changed section on RDF + added JSON-LD info

i didn't see reference to JSON-LD so thought about adding it here...

1. i would also like to add information how to use **multiple extensions** with the same database! (once i figure it out myself)
2. add invitation to develop **RDFa** and **Microdata** extensions... conservative folks could also work on **RDF/XML** :laughing: *(of course Microdata wouldn't go under RDF)*