Enforce CONTRIBUTING rules with ArchUnit

[pivovarit]

At the moment, contributing rules are paper-based with protein-based verification and enforcement - https://github.com/vavr-io/vavr/blob/master/CONTRIBUTING.md

We could enforce those using https://www.archunit.org

[bvkatwijk]

I think that ArchUnit does not support checking Javadoc.
An option for checking those rules could be CheckStyle