Adjust merge request template, recommend to merge fixes directly to stable

.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
 
+If your contribution is fixing an issue by a small change, please consider a merge into `stable` by using it as target branch.
+
 Related issue: #XXX <!-- it's okay to have no issue for small changes -->
 
 This Merge Request includes: