sublime-build: Adding documentation for cancelling Go Run

A small documentation change to let users know that when
using the Go Run command with a long-running process they
will need to use the custom 'Go: Cancel Build' command to
terminate the process. I also included an example of how to
create a key bindig for the command.

Change-Id: I0825193ba8db4225a6a44ad69025b39219e57086
Reviewed-on: https://go-review.googlesource.com/16507
Reviewed-by: Will Bond <will@wbond.net>
Reviewed-by: Jason Buberel <jbuberel@google.com>