Add a CONTRIBUTING.md

[MakisH]

This repository already has a .clang-format, integration with the system tests, etc. It would be nice to have all these as requirements/instructions in a CONTRIBUTING.md file, similar to the one in preCICE: https://github.com/precice/precice/blob/develop/docs/CONTRIBUTING.md

[davidscn]

The adapter follows (to my best knowledge) the deal.II contributing guideline. I would refer to their contributing file.

[MakisH]

This is also fine, but we need to document this somewhere.

Since the deal.II CONTRIBUTING.md does not mention how to run clang-format (or anything else that we may require), I would prefer to have some explicit instructions and anything else that we may require.