Add a PR template with a message about PRs needing to be for master

[ci skip]

Co-authored-by: Rémi Verschelde <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,11 @@
+<!--
+Pull requests should always be made for the `master` branch first, as that's
+where development happens and the source of all future stable release branches.
+
+Relevant fixes are cherry-picked for stable branches as needed.
+
+Do not create a pull request for stable branches unless the change is already
+available in the `master` branch and it cannot be easily cherry-picked.
+Alternatively, if the change is only relevant for that branch (e.g. rendering
+fixes for the 3.2 branch).
+-->