Document that now CUDS objects can have custom IRIs

[domgz]

I guess this fits either in the CUDS API tutorial or at the end of the ontology interface tutorial.

Some inspiration from the release text:

CUDS objects (ontology individuals) can now have a custom IRI, instead of one from the OSP-core namespace. Just use the irikeyword argument when calling the ontology class to create the CUDS object, e.g.city.Citizen(name='Klaus', age=55, iri='http://example.org/mynamespace#Klaus')`. However, in order to achieve optimal performance it is recommended to continue using the OSP-core namespace, as in such case OSP-core only has to store and handle an uuid, instead of an iri, which is usually far longer.`