doc: add contributing guide

Add a contributing guide as it is common in Open Source projects. The
guide explains the license and why signed commits are a requirement.

Signed-off-by: Jonas Remmert <jremmert@gmx.net>

CONTRIBUTING.md

@@ -0,0 +1,38 @@
+# Contributing
+
+Thank you for considering contributing to the lwm2m_server project! We
+appreciate your support and effort to improve this open-source project. Below
+are some guidelines to help you get started.
+
+## Making Changes and submitting a Pull Request
+
+1. **Commit Messages**: Use clear and descriptive commit messages. Follow the
+   convention:
+   ```plaintext
+   Component: Brief description of the change
+   ```
+2. **Push Your Changes**: Push your changes to your forked repository:
+   ```bash
+   git push origin my-feature-branch
+   ```
+2. **Open a Pull Request**: Navigate to the original repository and open a pull
+   request from your forked branch. Provide a clear and detailed description of
+   your changes.
+3. **Sign Your Commits**: Ensure all your commits are signed off. Use the
+   `--signoff` option when committing:
+   ```bash
+   git commit --signoff
+   ```
+
+## License
+
+By contributing to this project, you agree that your contributions are licensed
+under the [Apache License 2.0](LICENSE). This includes granting a patent
+license for your contributions as specified in Section 3 of the license.
+
+## Contact
+
+If you have any questions or need further assistance, feel free to open an
+issue on GitHub or contact the maintainers directly.
+
+Happy coding!