Add link to GTFS Resource and GTFS Realtime Specification

[e-lo]

To increase discoverability and knowledge of the GTFS Realtime best practices as well a their perceived legitimacy, it would be very helpful to have links to this repository from gtfs.mobilitydata.org as well as github.com/google/transit.

[Sergiodero]

As GTFS Best Practices (BP) are currently in the process of being merged to the specification, MobilityData is migrating outstanding issues and PRs from this repository to google/transit. Thus, this issue will be closed and further discussion regarding this BP should take place in google/transit. Please refer to Issue #421 for a more detailed explanation of the migration process and the proposed next steps.

With this, we’re hoping to bring more visibility to outstanding BP issues and to restart the discussion around them, so that any improvement that the community finds valuable could be carried forward.