Add gitter to readme

[philip-cline]

Checklist

• [x] Appropriate branch selected (all PRs must first be merged to dev before they can be merged to master)
• [x] Any modified or new methods or classes have helpful JavaDoc and code is thoroughly commented
• [x] The description lists all applicable issues this PR seeks to resolve
• [x] The description lists any configuration setting(s) that differ from the default settings
• [x] All tests and CI builds passing

Description

Adds link to Gitter room dedicated to GTFS-lib discussion. Slightly awkward since this is under the TRANSIT-data-tools space but since the TRANSIT-data-tools team is the main maintainer of this repo right now, this seems ok.

[philip-cline]

The gitter ones were not working, I have moved this to matrix links which should work.