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/3002Reviewed-by: Rob Pike <r@golang.org>

CONTRIBUTING.md

@@ -15,6 +15,8 @@ When filing an issue, make sure to answer these five questions:
 4. What did you expect to see?
 5. What did you see instead?
 
+General questions should go to the [golang-nuts mailing list](https://groups.google.com/group/golang-nuts) instead of the issue tracker.
+The gophers there will answer or ask you to file an issue if you've tripped over a bug.
 
 ## Contributing code