release MR template: add a note about merging stable changes back to

develop

.gitlab/merge_request_templates/Release.md

@@ -3,3 +3,4 @@
 * [ ]  Compile a changelog
 * [ ]  Create an MR with an announcement to pleroma.social
 * [ ]  Tag the release
+* [ ] Merge `stable` into `develop` (in case the fixes are already in develop, use `git merge -s ours --no-commit` and manually merge the changelogs)