This PR adds a comprehensive CONTRIBUTING.md file to provide a welcoming place for contributors to understand how to contribute to the stdlib-list project.

The new contribution guide includes:

• Welcoming introduction for new contributors with clear project overview
• Development setup instructions using the existing make dev workflow
• Code quality standards covering Ruff linting, MyPy type checking, and the 100% test coverage requirement
• Documentation build instructions using Sphinx
• Clear submission process with GitHub workflow guidelines
• Quick reference table of useful make commands for easy access
• Links to existing documentation including the module inclusion policy
• Project history context acknowledging the original maintainers and project transfer

The guidelines document the existing well-established development infrastructure without changing any code, making this a minimal but complete addition. All documented commands have been validated to work correctly with the current project setup.

This provides new contributors with everything they need to get started quickly while maintaining the project's high standards for code quality, testing, and documentation.

Fixes #165.

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