To make those two new communication channels that are up for trial more
discoverable beyond the RFC sent to llvm-dev and cfe-dev, also add them
to the section where mailing lists and IRC are advertised.
I am a bit concerned about going too fast here: while I'm supportive of replacing mailing-lists with online forums and IRC with a more modern solution which includes channels and richer content, the proposal is really new. The changes in this patch aren't even mentioning that this is an experiment or anything, it even put Discord *ahead* of IRC as far as I can tell (apologize if my HTML internal rendering is missing anything).