Document the method from_iri #168

Sometimes it's desired to get a CUDS object by an IRI without knowing a priori the namespace.
For this, the method from_iri is useful. However, it's not documented.

Example:

from osp.core.namespaces import from_iri
oentity = from_iri("http://www.osp-core.com/city#Citizen")

Note: To get a CUDS object, the method session.load_from_iri can be used.