github: update pull request template
The current wording is a bit curt. Flesh it out a bit and put in some
useful detail.
-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://open-power.github.io/skiboot/
+Hi there,
+
+The skiboot project (generally) doesn't take contributions via github pull
+requests. Contributions should sent to the skiboot mailing list using the
+git-format-patch and git-send-email tools. If you're unfamiliar with these
+tools, there's a guide here:
+
+https://www.freedesktop.org/wiki/Software/PulseAudio/HowToUseGitSendEmail/
+
+The list address is:
+
+ skiboot@lists.ozlabs.org
+
+Posting to the list does not require subscribing to the list. Posts from a
+non-subscribers are moderated to cut down on spam, but anyone posting
+genuine contributions will be whitelisted.
+
+See https://open-power.github.io/skiboot/ for more details.
