.github: add a pull request warning that explains the mailing list workflow

Signed-off-by: Thomas Devoogdt <thomas@devoogdt.com>
Reviewed-by: J. Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

.github/pull_request_template.md

@@ -0,0 +1,6 @@
+Please do not submit a Pull Request via GitHub. Buildroot makes use of a
+[mailing list](http://lists.buildroot.org/mailman/listinfo/buildroot) for patch submission and review.
+See [submitting your own patches](http://buildroot.org/manual.html#submitting-patches) for more info.
+
+Thanks for your help!
+