Update contributor's guide (#502)

Co-authored-by: Johannes Schmölder <[email protected]>

Author: jbreue16

CONTRIBUTING.md

@@ -1,8 +1,46 @@
 ## Contributing
 
 CADET-Core is an open-source project and we are very happy to accept contributions from the community.
-Please feel free to open issues or submit patches (preferably as pull requests) any time.
-For planned larger contributions, it is often beneficial to get in contact with us first, e.g. through the [CADET forum](https://forum.cadet-web.de/).
+Please feel free to open issues or submit patches (preferably as pull requests) at any time.
+For planned larger contributions, we would encourage you to get in touch with us first, e.g. through the [CADET forum](https://forum.cadet-web.de/).
 
-For detailed information on how to contribute, including guidelines, coding standards, and best practices, please refer to our [Developer Guide](https://cadet.github.io/master/developer_guide/index.html).
+For detailed information on how to contribute, including installation, dependencies, code style, testing, publishing, or model expansion, please refer to our [Developer Guide](https://cadet.github.io/master/developer_guide/index.html).
 
+## Issues & Pull Requests
+
+Follow our [issue and PR templates](.github/ISSUE_TEMPLATE/) and [code of conduct](CODE_OF_CONDUCT.md).
+
+### Issues vs. Pull Requests
+
+We use **GitHub issues** for:
+
+* Reporting bugs
+* Suggesting new features
+* Discussing technical problems and ideas
+
+Please use an appropriate provided template and ensure your description and instructions are clear.
+
+**Pull Requests** should be submitted after your changes are complete and tested.
+Make sure your PR addresses one issue or feature to keep reviews manageable.
+Reference related issues in your PR description.
+
+## Branch Policy
+
+We use a branch naming convention to keep contributions organized:
+
+* **Feature branches**: `feature/your-feature-name`
+* **Bug fixes**: `fix/short-description`
+* **Hotfixes**: `hotfix/issue-description`
+* **Documentation updates**: `docs/update-description`
+
+## Contributor License Agreement
+
+Review our [CLA](CLA.md) before submitting your first PR.
+
+## Documentation
+
+Steps for building the documentation are described in [doc/README.md](doc/README.md).
+
+## License
+
+This project is licensed under the [GPL-3.0 License](LICENSE.txt).