docs: add CONTRIBUTING.md guide

CONTRIBUTING.md

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+# Contributing to Search-R1
+
+Thank you for your interest in contributing to Search-R1!
+
+## Development Setup
+
+1. **Requirements**
+   - Python 3.10+
+   - Git
+
+2. **Clone the repository**
+   ```bash
+   git clone https://github.com/PeterGriffinJin/Search-R1.git
+   cd Search-R1
+   ```
+
+3. **Create a virtual environment**
+   ```bash
+   python -m venv venv
+   source venv/bin/activate
+   ```
+
+4. **Install dependencies**
+   ```bash
+   pip install -e .
+   pip install -e ".[dev]"  # if available
+   ```
+
+5. **Run tests**
+   ```bash
+   pytest tests/
+   ```
+
+## Making Changes
+
+1. **Create a feature branch**
+   ```bash
+   git checkout -b feat/your-feature-name
+   ```
+
+2. **Code style**
+   - Follow PEP 8
+   - Use type hints
+   - Add docstrings
+
+3. **Commit and push**
+   ```bash
+   git commit -m "feat: add your feature"
+   git push origin feat/your-feature-name
+   ```
+
+## Pull Request Process
+
+1. Fork the repository
+2. Create your feature branch
+3. Make your changes with tests
+4. Submit a PR with description
+
+## Reporting Issues
+
+- Use GitHub Issues for bugs and feature requests
+- Include Python version and OS
+- Provide reproduction steps
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the project license.