Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to HohmannHET
+
+Thank you for your interest in HohmannHET! This document explains how to
+contribute effectively.
+
+## Tri-Language Parity Rule
+
+Every physics function must be implemented identically in Python, C++, and
+MATLAB. If you change a formula or add a feature in one language, you must
+update all three. Cross-language numerical agreement must hold to `1e-6`.
+
+## Development Setup
+
+### Python
+```bash
+cd python
+pip install -e ".[dev]"
+pytest tests/ -v
+```
+
+### C++
+```bash
+cd cpp
+cmake -B build -DCMAKE_BUILD_TYPE=Release
+cmake --build build
+cd build && ctest --output-on-failure
+```
+
+### MATLAB
+```matlab
+cd matlab/tests
+results = runtests({'TestDynamics','TestPropulsion','TestOptimization'});
+table(results)
+```
+
+## Pull Request Checklist
+
+- [ ] All three language implementations are updated (if applicable)
+- [ ] Tests pass in all languages
+- [ ] New functions include docstrings / Doxygen / help blocks
+- [ ] Physical equations are documented in LaTeX notation
+- [ ] Constants match the shared reference values in the README
+
+## Reporting Issues
+
+Please include:
+- Which language implementation is affected (Python / C++ / MATLAB / all)
+- Minimal code to reproduce the issue
+- Expected vs. actual output
+- Your environment (OS, compiler/Python version, MATLAB release)
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the
+Apache License 2.0, consistent with the project license.