Create support caveat

[baentsch]

As originally proposed in #2033 (comment) add wording in suitable places making consumers aware of support limitations.

This project is not commercially supported. All guidelines and goals stated below are reflections of current practices, executed by a community of voluntary contributors on a best effort basis and may change at any time.

Any entity seeking more reliable commitments is strongly encouraged to join our community and thus enhance the code and support that the community can provide.

Locations where to add this: README.md, SECURITY.md, www. Others?

Re-phrasing proposals via comments to this issue before this gets deployed.