message: README: Mention mailing list
diff: @@ -14,6 +14,11 @@ This implementation is in progress, and things will keep moving around in the
  near future, so it's not yet a good time to contribute, and it's definitely not
  a good time to rely on it.
  +To discuss the spec or other age related topics, please email the mailing list
  +at age-dev@googlegroups.com. Subscribe at
  +[groups.google.com/d/forum/age-dev](https://groups.google.com/d/forum/age-dev)
  +or by emailing age-dev+subscribe@googlegroups.com.
  +
  ## License
  Licensed under either of
  