add CONTRIBUTING.md

Author: PierreMarchand20

CONTRIBUTING.md

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+# Contributing
+
+Thank you for considering contributing to **Htool-DDM**! We welcome bug reports, feature suggestions, and pull requests.
+
+## Opening issues
+
+If you encounter a bug or have a question, please [open an issue](https://github.com/htool-ddm/htool/issues).
+When reporting a bug, include:
+
+- a clear description of the problem,
+- the expected behavior,
+- a minimal, reproducible example (if possible),
+- your environment (compiler, OS, library versions).
+
+## Submitting changes (Pull Requests)
+
+To learn how to submit a pull request, please refer to GitHub’s  
+[pull request documentation](https://docs.github.com/fr/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests).
+
+Before your pull request can be merged into `main`:
+
+1. **Tests must run successfully.**
+2. **Code style and formatting must follow project standards.**
+
+Instructions for running tests locally and applying formatting can be found in the  
+[Htool developer guide](https://htool-ddm.pages.math.cnrs.fr/cpp_api/developer_guide.html#developer-guide).
+
+## Code Style and Guidelines
+
+- Follow the project’s formatting rules (see [developer guide](https://htool-ddm.pages.math.cnrs.fr/cpp_api/developer_guide.html#formatting)).
+- Write clear commit messages.
+- Keep pull requests focused and reasonably small.
+
+---
+
+Thank you again for helping improve Htool-DDM!