Include final mailing list links

Thanks to @emilypi, we now have worked out the details of our mailing list. I have updated the communications.md document accordingly.

As a matter of good hygiene, I'd love a quick review before posting this. There is no change of policy here -- just a simplification of provider (we already use Google for lots of other business). If it all looks good to one of you, I'll just blast this out.