Mention RDF/JS spec compliance

[joepio]

The API of @ontologies/core seems to mimic / implement the RDF/JS DataModel. If this library is to be mentioned in http://rdf.js.org/, it should specify compliance.

[rescribet]

That's on purpose, but in the end it is up to the factory you're using, which changes concrete implementation.

RDF/JS data model is too constraining IMO, I'd like the next version to include some more methods on the factory so it has a full functional interface, removing the need for library code to know about things like termType: "Literal", or some mysterious empty string property on quad (my programmer instincts tell me someone wanted to union Terms and Quad 🤷🏻‍♂️).