Document the source of  GTFS Schedule Compliance Check and GTFS RT Compliance Check

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when I don't know where a line in the compliance checks comes from.

**Describe the solution you'd like**
Try to document in english, not in code - each line and say where in airtable or GTFS the line comes from.

We can start documenting it in this issue and then make it a part of a README.