Add CONTRIBUTING.md with community contribution guidelines

CONTRIBUTING.md

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+# Contributing to Git Developer Pages
+
+Thank you for your interest in contributing to [git.github.io](https://git.github.io),
+the community website for Git development, home to the
+[Git Rev News newsletter](https://git.github.io/rev_news/),
+[Hacking Git](https://git.github.io/Hacking-Git/), and mentoring program
+information.
+
+---
+
+## Table of Contents
+
+- [Ways to Contribute](#ways-to-contribute)
+- [Submitting Changes via Pull Request](#submitting-changes-via-pull-request)
+- [Submitting Changes via Email](#submitting-changes-via-email)
+- [Running the Site Locally](#running-the-site-locally)
+- [Writing Guidelines](#writing-guidelines)
+- [Code Style](#code-style)
+- [Git Rev News Newsletter](#git-rev-news-newsletter)
+- [Reporting Issues](#reporting-issues)
+
+---
+
+## Ways to Contribute
+
+In general we accept all kind of contributions to all the pages on
+this website, but here are some suggestions along with a few
+guidelines:
+
+- Fix typos, broken links, or outdated information across the site
+  (except for already published editions of Git Rev News, which are
+  kept as historical archives). If possible, open a Pull Request (PR)
+  to submit your changes.
+- Improve or add content to sections of the website such as the
+  Links page, Mentoring pages, or general documentation (opening a PR
+  is preferred for these as well).
+- Add entries or articles to the upcoming edition of the Git Rev News
+  newsletter (see [below](#git-rev-news-newsletter)).
+- Improve site infrastructure (CSS, JavaScript, Jekyll layouts, or
+  plugins). Before starting work on major improvements, please check
+  [existing issues](https://github.com/git/git.github.io/issues) and
+  open a new issue for discussion first.
+- Participate in community discussions by sharing opinions on
+  [open issues](https://github.com/git/git.github.io/issues) or
+  reviewing [open Pull Requests](https://github.com/git/git.github.io/pulls).
+- Report bugs, broken links, or suggestions via
+  [GitHub Issues](https://github.com/git/git.github.io/issues).
+
+---
+
+## Submitting Changes via Pull Request
+
+1. **Fork** this repository on GitHub.
+2. **Clone** your fork locally:
+   ```sh
+   git clone https://github.com/<your-username>/git.github.io.git
+   cd git.github.io
+   ```
+3. **Create a branch** for your changes:
+   ```sh
+   git checkout -b my-fix
+   ```
+4. **Make your changes**, following the [Writing Guidelines](#writing-guidelines)
+   below.
+5. **Test locally** if needed by [running the site](#running-the-site-locally)
+   and verifying your changes look correct. (Note: this is not required
+   for newsletter contributions or small changes like typofixes.)
+6. **Commit** with a clear, descriptive message:
+   ```sh
+   git add .
+   git commit -m "Brief description of the change"
+   ```
+   For Git Rev News contributions, use the `rn-XXX: ` prefix for your
+   commit subject (e.g. `rn-114: Add link to new Git tutorial`).
+7. **Push** to your fork and open a Pull Request against the `master`
+   branch of this repository.
+
+> **Note:** Keep PRs focused on a single topic. Avoid mixing unrelated
+> changes (such as infrastructure changes with content updates).
+
+---
+
+## Submitting Changes via Email
+
+If you prefer not to use GitHub, you can send patches by email to the
+maintainers:
+
+- **Christian Couder**: <christian.couder@gmail.com>
+- **Kaartic Sivaraam**: <kaartic.sivaraam+git@gmail.com>
+
+You may also CC the Git mailing list at <git@vger.kernel.org> if
+appropriate.
+
+---
+
+## Running the Site Locally
+
+This site is built with [Jekyll](https://jekyllrb.com/) and hosted via
+GitHub Pages.
+
+Note: Running the site locally is helpful when working on layouts,
+styles, or general pages, but it is **not required** when submitting
+content for Git Rev News editions or sending typofixes and similar
+small changes.
+
+### Prerequisites
+
+- Ruby (2.x or later) with development packages
+- Bundler
+
+### Steps
+
+```sh
+# Install system dependency (Ubuntu/Debian)
+sudo apt-get install zlib1g-dev
+
+# Install Bundler gem
+sudo gem install bundler
+
+# Clone the repo and install dependencies
+git clone https://github.com/git/git.github.io.git
+cd git.github.io
+bundle install
+
+# Start the local server
+bundle exec jekyll serve
+```
+
+Then open your browser at <http://localhost:4000>.
+
+See [GitHub Pages documentation](https://help.github.com/articles/using-jekyll-with-pages/)
+for more details.
+
+---
+
+## Writing Guidelines
+
+- Use clear, concise language suitable for a developer audience.
+- Write content in **Markdown** (`.md` or `.markdown` files).
+- Keep lines wrapped at around 72 characters when possible.
+- Use relative links within the site where possible.
+- When adding code examples, use fenced code blocks with a language
+  tag:
+  ````
+  ```sh
+  git log --oneline
+  ```
+  ````
+  Supported languages for syntax highlighting include: `sh`/`bash`,
+  `c`, `diff`, `go`, `ini`, `python`, `ruby`, `yaml`, and others.
+- Avoid adding unnecessary dependencies or large binary assets.
+
+---
+
+## Code Style
+
+- **JavaScript / CSS:** Keep changes minimal and consistent with the
+  existing style in `script/` and `css/`.
+- **Jekyll Layouts & Plugins:** Changes to `_layouts/`, `_includes/`, or
+  `_plugins/` should be well-tested locally before submitting.
+- **Gemfile:** Do **not** modify the `Gemfile` unless there is a
+  clear, necessary reason, as it is shared and affects all
+  contributors' environments.
+- **Whitespace:** Prefer Unix line endings (LF). Avoid trailing
+  whitespace.
+
+---
+
+## Git Rev News Newsletter
+
+The [Git Rev News](https://git.github.io/rev_news/) newsletter is
+published monthly.
+
+### How to Contribute Content
+
+Before being published, an upcoming edition is prepared in advance
+as a draft Markdown file under `rev_news/drafts/` (e.g.
+`rev_news/drafts/edition-XXX.md`). When an edition is published, its
+file is moved under `_posts/` and renamed (e.g.
+`_posts/YYYY-MM-DD-edition-XXX.markdown`).
+
+You can contribute in several ways:
+- **Share entries in the edition issue:** A dedicated GitHub Issue is
+  opened for every upcoming edition, titled `"Any comment about
+  upcoming Git Rev News edition XXX"` (where `XXX` is the edition
+  number; for example, see [Issue #860](https://github.com/git/git.github.io/issues/860)).
+  You can comment directly on that issue with links, articles,
+  tools, or news you would like included.
+- **Submit a Pull Request:** Add your entry directly to the draft file
+  under `rev_news/drafts/` for the upcoming edition. Common entries
+  include:
+  - Links and short descriptions for tutorials, articles, or blog posts.
+  - Recent Git-related releases, tools, or projects.
+  - Discussions from the Git mailing list.
+  - Lighthearted Git-related content.
+- **Submission guidelines:** Check the
+  [news sources](https://git.github.io/rev_news/news_sources/) page for
+  more suggestions on finding news items.
+- **Email:** Contact the maintainers via email (see
+  [above](#submitting-changes-via-email)).
+
+### Newsletter Guidelines
+
+- **Do not edit past editions:** Already published editions are
+  archived historical records and should not be modified.
+- **No local testing required:** You do not need to install Ruby or
+  build the site locally when submitting newsletter entries.
+- **Commit prefix:** Format your commit message subject starting with
+  `rn-XXX: ` where `XXX` is the edition number (e.g.
+  `rn-114: Add link to Git tutorial`).
+
+---
+
+## Reporting Issues
+
+Found a bug, broken link, or outdated content? Please
+[open an issue](https://github.com/git/git.github.io/issues/new) on
+GitHub with:
+
+- A clear title describing the problem.
+- The URL of the affected page (if applicable).
+- Steps to reproduce or a description of what you expected vs. what
+  you saw.
+
+---
+
+We appreciate every contribution, big or small. Thank you for helping
+improve the Git community website!