Unstar mapping: PR ready

Dear all,

per my action [1], I made a PR on RDF-Concepts to add a section defining 
a (currently non-normative) unstar mapping.

   https://github.com/w3c/rdf-concepts/pull/115


Feel free to review and comment. Time permitting, we can discuss this 
tomorrow.

   best

[1] https://github.com/w3c/rdf-concepts/pull/115