.github: add SUPPORT file

GitHub recently added support for a SUPPORT file:

https://github.com/blog/2400-support-file-support

This SUPPORT file is a very lightly edited copy
of the wiki entry on asking questions:

https://github.com/golang/go/wiki/Questions

Change-Id: Ic1eb74d985ea30862defb99750fb42da84e492de
Reviewed-on: https://go-review.googlesource.com/50930Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.github/SUPPORT

+Unlike many projects on GitHub, the Go project does not use its bug tracker for general discussion or asking questions.
+We only use our bug tracker for tracking bugs and tracking proposals going through the [Proposal Process](https://golang.org/s/proposal-process).
+
+For asking questions, see:
+
+* [The golang-nuts mailing list](https://groups.google.com/d/forum/golang-nuts)
+
+* [The Go Forum](https://forum.golangbridge.org/), a web-based forum
+
+* [Gophers Slack](https://gophers.slack.com), use the [invite app](https://invite.slack.golangbridge.org/) for access
+
+* [Stack Overflow](http://stackoverflow.com/questions/tagged/go) with questions tagged "go"
+
+* **IRC** channel #go-nuts on Freenode