github: azure: Update our GitHub template to note for CI

While the general policy of not taking changes to the project via pull
requests directly on GitHub has not changed, it can be useful to submit
a PR there in order to trigger a CI run on Azure.  These are run
automatically and the results are populated back to GitHub.  Add a note
to the template to reflect this.
Signed-off-by: NTom Rini <trini@konsulko.com>

.github/pull_request_template.md

 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
+
+The only exception to this is in order to trigger a CI loop on Azure prior
+to posting of patches.