link to "How Transit integrates GTFS" docs

[derhuerst]

The How Transit integrates GTFS may be Transit-specific, but IMO they serve as a great collection of best practices on top of the GTFS Best Practices (which they also link to), so we could link to them here.

[barbeau]

@derhuerst Sounds good to me! Maybe add them under the existing GTFS Best Practices entry, with a clear indication that it's by Transit App? PRs welcome!