Undocumented versioning system

[MakisH]

In the OpenFOAM adapter, we follow semantic versioning, and we have defined the adapter config as the API that defines whether we have a breaking version or not: https://precice.org/adapter-openfoam-get.html#what-does-the-adapter-version-mean

In the CalculiX adapter, we have decided that we are following the version of CalculiX that we support, thus we have v2.19.0, v2.20.0, etc. But we have not defined (or, most importantly, documented), what the third digit means and when it changes.

We need a concrete statement in the README.