Draft of usage note

I made a draft of the usage note in response to Dave McComb's comment, and
I wrote to him to socialize that.

Here is my draft.  It is quite short, and modeled on the RDFS paragraph
that's already there.  Since we *aren't* using OWL inferencing, it is a lot
shorter.


A related topic - the only reference to OWL that we make in shacl.ttl is to
owl:Ontology.  Is there a way to make SHACL completely non-dependent on
OWL?   Do we even need to declare an owl:Ontology in shacl.ttl?  Part of
the motivation for not declaring things as annotation properties is that we
aren't using OWL.  And we really aren't; we never actually define any
entity that is a member of a class in the OWL namespace (except the one
owl:Ontology at the top).


Dean