message: Update CONTRIBUTING.md
  Added changelog requirements
diff: - Develop your code
  - Commit and push in regular intervalls
  - Run the tests locally
  +- Add a line to the [CHANGELOG.md](https://github.com/h-da/geli/blob/develop/CHANGELOG.md) under the "NEXT" section
  - Open a Pull-Request
  - If CI/Coverage give their OK we can merge
  - The PR gets merged to `develop`, which will push a new Docker-Image-Version with the tag `develop` and `latest`
  