docs: add CONTRIBUTING.md with development guidelines and setup instructions

CONTRIBUTING.md

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+# Contributing to PR-Agent
+
+Thank you for your interest in contributing to the PR-Agent project! 
+
+## Getting Started
+
+1. Fork the repository and clone your fork
+2. Install Python 3.10 or higher
+3. Install dependencies (`requirements.txt` and `requirements-dev.txt`)
+4. Create a new branch for your contribution:
+   - For new features: `git checkout -b feature/your-feature-name`
+   - For bug fixes: `git checkout -b fix/issue-description`
+5. Make your changes
+6. Write or update tests as needed
+7. Run tests locally to ensure everything passes
+8. Commit your changes using conventional commit messages
+9. Push to your fork and submit a pull request
+
+## Development Guidelines
+
+- Keep pull requests focused on a single feature or fix
+- Follow the existing code style and formatting conventions
+- Add unit tests for any new functionality using pytest
+- Ensure test coverage for your changes
+- Update documentation as needed
+
+## Pull Request Process
+
+1. Ensure your PR includes a clear description of the changes
+2. Link any related issues
+3. Update the README.md if needed
+4. Wait for review from maintainers
+
+## Questions or Need Help?
+
+- Join our [Discord community](https://discord.com/channels/1057273017547378788/1126104260430528613) for questions and discussions
+- Check the [documentation](https://qodo-merge-docs.qodo.ai/) for detailed information
+- Report bugs or request features through [GitHub Issues](https://github.com/qodo-ai/pr-agent/issues)