Update or remove CONTRIBUTING.md

[matrss]

The "Pull Request Process" section of that file (which is more than half of it) makes no sense at all. The other part makes sense, but doesn't really help in starting to contribute.

In the spirit of DRY it would be better to simply link to https://open-mss.github.io/contributing/, I think.

[ReimarBauer]

linking to the pages is better. There we need to add a hint to SemVer.