Add a hint to merge requests template about rebases

.gitlab/merge_request_templates/Merge request.md

+Please avoid merging the base branch into your feature branch. We are working with rebases and those merged tend to cause trouble. 
+For further questions, join us at Matrix: https://matrix.to/#/#funkwhale-dev:matrix.org
+
 Related issue: #XXX <!-- it's okay to have no issue for small changes -->
 
 This Merge Request includes: