doc: direct people to the mailing list

Since the move to Github, we've started to receive lots of
introductory questions to the bug tracker. I posit this is because
most projects on Github don't have mailing lists, so the culture on
Github is to use the Issue Tracker as a discussion forum.

The Go project doesn't use the Issue Tracker as our first point of
communication. This CL updates CONTRIBUTING.md (which is linked when
you file a bug or send a pull request), to mention that we have a
mailing list.

It certainly won't stop all the errant bug reports, but it should
help.

Change-Id: Id8fbfd35b73f5117617dff53b1e72d5b5276388b
Reviewed-on: https://go-review.googlesource.com/3002


Reviewed-by: Rob Pike <r@golang.org>