[GTFS] Link back to transit vocabulary

[pietercolpaert]

In current existing projects, the transit (https://github.com/iand/vocab-transit) vocabulary is used, which is not maintained anymore [1]. It would be helpful to add owl:sameAs links in the GTFS's RDF specification.

Furthermore, since we have decided to create the GTFS LD vocabulary as a response to transit vocabulary not being used anymore, and not being 1 on 1 map-able from GTFS-CSV, we should still refer to the transit vocabulary in the human readable docs.

Reported by @rtroncy during ISWC2014's semdev challenge

[rtroncy]

owl:sameAs or other OWL constructs such as owl:equivalentClass or owl:equivalentProperty between the LinkedGTFS and Transit vocabularies