Add a github template telling people to not use pull requests there

On our mirror account on github we have gotten a handful of pull
requests.  At this time github does not allow you to disable pull
requests on a project.  The generally suggested work-around is to add a
pull request template that tells people to not use that workflow.  Add
one here that points to the wiki page on submitted patches.

Signed-off-by: Tom Rini <trini@konsulko.com>