.github: recommend 'go bug' when filing an issue

It was released with 1.8, so it should be widely available.

Change-Id: I7dcf205009b528071ea63f99cb5da0db183df341
Reviewed-on: https://go-review.googlesource.com/50932Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CONTRIBUTING.md

@@ -15,6 +15,7 @@ the behavior you are seeing is confirmed as a bug or issue, it can easily be re-
 Sensitive security-related issues should be reported to [security@golang.org](mailto:security@golang.org).
 See the [security policy](https://golang.org/security) for details.
 
+The recommended way to file an issue is by running `go bug`.
 Otherwise, when filing an issue, make sure to answer these five questions:
 
 1. What version of Go are you using (`go version`)?