chore: add CONTRIBUTING.md for RSR compliance

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+<!-- SPDX-License-Identifier: PMPL-1.0-or-later -->
+# Contributing
+
+Contributions are welcome! Please:
+
+1. Fork the repository
+2. Create a feature branch from `main`
+3. Ensure all CI checks pass
+4. Submit a pull request
+
+## Standards
+
+This project follows the [Rhodium Standard Repository (RSR)](https://github.com/hyperpolymath/standards) conventions.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under PMPL-1.0-or-later (with MPL-2.0 as automatic legal fallback).