It is not often clear that the issues must be prefixed with x/mobile.
Provide a template to give a hint to the user that all mobile bugs
must be prefix relative to golang.org base.

Change-Id: Ia5b35db529798a230850d935ff3e0e800d1948f9
Reviewed-on: https://go-review.googlesource.com/15401
Reviewed-by: Andrew Gerrand <adg@golang.org>