Update contributing guide for project based on community landing page

[degenaro]

Issue description / feature objectives

The README.md for the compliance-trestle leg of oscal-compass should be commensurate with the community landing page at https://github.com/oscal-compass/community

Caveats / Assumptions

The community page comprises contributions guidelines and such. Link from compliance-trestle to community page as appropriate.

Completion Criteria

Any duplicated/conflicting info is eliminated from compliance trestle repo.