Add CONTRIBUTING.md file with comprehensive contribution guidelines

CONTRIBUTING.md

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+# Contributing to stdlib-list
+
+Thank you for your interest in contributing to `stdlib-list`! We welcome contributions from everyone.
+
+## Overview
+
+`stdlib-list` provides lists of Python standard library modules for Python versions 2.6 through 3.13. The project maintains high standards for code quality, documentation, and testing.
+
+## Development Setup
+
+We use a `Makefile` to streamline the development workflow. To get started:
+
+1. **Clone the repository:**
+   ```bash
+   git clone https://github.com/pypi/stdlib-list.git
+   cd stdlib-list
+   ```
+
+2. **Set up the development environment:**
+   ```bash
+   make dev
+   ```
+   This creates a Python virtual environment and installs all development dependencies.
+
+## Code Quality Standards
+
+We maintain high code quality standards using automated tools:
+
+### Linting and Formatting
+
+- **Ruff**: Used for code linting and formatting
+- **MyPy**: Used for static type checking
+
+Run linting checks:
+```bash
+make lint
+```
+
+Auto-format code:
+```bash
+make reformat
+```
+
+### Testing
+
+We require **100% test coverage** for all code changes.
+
+Run tests:
+```bash
+make test
+```
+
+If you're working on a specific test, you can run individual tests:
+```bash
+make test TESTS="test_name_pattern"
+```
+
+## Documentation
+
+Documentation is built using Sphinx and hosted at [pypi.github.io/stdlib-list](https://pypi.github.io/stdlib-list/).
+
+Build documentation locally:
+```bash
+make doc
+```
+
+### Module Inclusion Policy
+
+Please read our [module inclusion policy](docs/module-policy.rst) to understand how we determine which modules should be included in the standard library lists.
+
+## Making Contributions
+
+### Types of Contributions
+
+We welcome several types of contributions:
+
+1. **Bug reports**: If you find a missing or incorrectly included module
+2. **Bug fixes**: Corrections to module lists or code issues
+3. **Documentation improvements**: Clarifications or additions to docs
+4. **New Python version support**: Adding support for new Python releases
+
+### Submission Process
+
+1. **Fork the repository** on GitHub
+2. **Create a feature branch** from `main`:
+   ```bash
+   git checkout -b feature/your-feature-name
+   ```
+3. **Make your changes** following our code quality standards
+4. **Run the full test suite** to ensure nothing is broken:
+   ```bash
+   make lint && make test
+   ```
+5. **Commit your changes** with a clear, descriptive commit message
+6. **Push to your fork** and **create a pull request**
+
+### Pull Request Guidelines
+
+- Provide a clear description of the problem and solution
+- Include tests for any new functionality
+- Ensure all CI checks pass
+- Keep changes focused and atomic
+- Reference any related issues
+
+### Code Style
+
+- Follow Python PEP 8 (enforced by Ruff)
+- Use type hints (checked by MyPy)
+- Write clear, descriptive commit messages
+- Add docstrings for new functions and classes
+
+## Development Workflow Commands
+
+Here's a quick reference of useful `make` commands:
+
+| Command | Purpose |
+|---------|---------|
+| `make dev` | Set up development environment |
+| `make lint` | Run linting and type checking |
+| `make reformat` | Auto-format code with Ruff |
+| `make test` | Run full test suite with coverage |
+| `make doc` | Build documentation |
+| `make package` | Build distribution packages |
+
+## Getting Help
+
+- **Issues**: For bug reports and feature requests, please use [GitHub Issues](https://github.com/pypi/stdlib-list/issues)
+- **Discussions**: For questions about usage or contribution ideas
+- **Documentation**: Check our [online documentation](https://pypi.github.io/stdlib-list/)
+
+## Project History
+
+This project was originally created by [@jackmaney](https://github.com/jackmaney) and later transferred to the PyPI organization with new maintainers. We appreciate all contributions that help maintain this valuable resource for the Python community.
+
+Thank you for contributing! 🐍