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>
diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
new file mode 100644
index 0000000..23599b9
--- /dev/null
+++ b/.github/pull_request_template.md
@@ -0,0 +1,3 @@
+Please do not submit a Pull Request via github.  Our project makes use of
+mailing lists for patch submission and review.  For more details please
+see https://www.denx.de/wiki/U-Boot/Patches