Add doctest example about how subclassing Resource works
Add doctest example about how subclassing Resource works

marma/rdflib,avorio/rdflib,yingerj/rdflib,armandobs14/rdflib,armandobs14/rdflib,ssssam/rdflib,dbs/rdflib,RDFLib/rdflib,marma/rdflib,RDFLib/rdflib,marma/rdflib,dbs/rdflib,avorio/rdflib,dbs/rdflib,armandobs14/rdflib,ssssam/rdflib,avorio/rdflib,marma/rdflib,RDFLib/rdflib,yingerj/rdflib,dbs/rdflib,armandobs14/rdflib,avorio/rdflib,RDFLib/rdflib,ssssam/rdflib,ssssam/rdflib,yingerj/rdflib,yingerj/rdflib