📝 docs(guide): add CONTRIBUTING.md

- introduce guidelines for contributing to cull-gmail
- include sections on code of conduct, how to contribute, and development setup
- provide coding standards, commit message conventions, and testing requirements
- explain licensing terms for contributions under MIT License

CONTRIBUTING.md

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+# Contributing to cull-gmail
+
+Thank you for your interest in contributing to cull-gmail! We welcome contributions from the community and appreciate your help in making this project better.
+
+## Code of Conduct
+
+By participating in this project, you agree to maintain a respectful and inclusive environment for all contributors.
+
+## How to Contribute
+
+### Reporting Bugs
+
+If you find a bug, please open an issue on GitHub with:
+- A clear, descriptive title
+- Steps to reproduce the issue
+- Expected behaviour vs actual behaviour
+- Your environment (OS, Rust version, etc.)
+- Any relevant error messages or logs
+
+### Suggesting Features
+
+Feature requests are welcome! Please open an issue describing:
+- The problem you're trying to solve
+- Your proposed solution
+- Any alternative solutions you've considered
+
+### Pull Requests
+
+1. **Fork the repository** and create your branch from `main`
+2. **Make your changes**, following the coding standards below
+3. **Add tests** if applicable
+4. **Ensure all tests pass** by running `cargo test`
+5. **Update documentation** as needed
+6. **Format your code** with `cargo fmt`
+7. **Run the linter** with `cargo clippy`
+8. **Commit your changes** with clear, descriptive commit messages
+9. **Submit a pull request** with a description of your changes
+
+## Development Setup
+
+```bash
+# Clone your fork
+git clone https://github.com/your-username/project-name.git
+cd project-name
+
+# Build the project
+cargo build
+
+# Run tests
+cargo test
+
+# Format code
+cargo fmt
+
+# Run linter
+cargo clippy
+```
+
+## Coding Standards
+
+- Follow the official [Rust Style Guide](https://doc.rust-lang.org/nightly/style-guide/)
+- Use `cargo fmt` to format code
+- Ensure `cargo clippy` passes without warnings
+- Write clear, self-documenting code with appropriate comments
+- Add documentation comments for public APIs
+- Keep functions focused and reasonably sized
+- Write tests for new functionality
+
+## Commit Messages
+
+- Use clear, descriptive commit messages
+- Use conventional commits 
+- Start with a verb in the present tense (e.g., "Add", "Fix", "Update")
+- Keep the first line under 50 characters
+- Add a detailed description if necessary
+
+## Testing
+
+- Write unit tests for new functionality
+- Ensure all existing tests pass
+- Aim for meaningful test coverage
+- Test edge cases and error conditions
+
+## Documentation
+
+- Update the README if your changes affect usage
+- Add inline documentation for public APIs
+- Include examples in doc comments where helpful
+
+## PRLOG and CHANGELOG
+
+ - The Pull Request log is updated automatically by CI.
+ - The Changelog is generated automatically from relevant conventional commits by CI
+
+## License
+
+By contributing to cull-gmail, you agree that your contributions will be licensed under the MIT License. This means:
+
+- You grant permission for your contributions to be used, modified, and distributed under the terms of the MIT License
+- You confirm that you have the right to submit the code under this license
+- You understand that your contributions will become part of the project and available to all users under the MIT License
+
+## Questions?
+
+If you have questions about contributing, feel free to open an issue or reach out to the maintainers.
+
+Thank you for contributing!