Remove stale Gitter link; maybe link to Slack

[tordans]

8 years ago this sentence was added to the Readme:

Ongoing and past converations about the project take place in the project's Gitter chat room, and periodic updates go out on the OAM-Talk mailing list.)

Looking at the Gitter thread, I think this communication channel went stale. I suggest to officially close it on Gitter and remove it from the readme.

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Maybe linking to the HOTOSM Slack channel is better? (Including instructions on how to get in.)