Add Merge request template

.gitlab/merge_request_templates/merge_request.md

+Please fill in the appropriate checklist below (delete whatever is not relevant).
+These are the most common things requested on pull requests (PRs).
+
+## PR checklist
+ - [ ] This comment contains a description of changes (with reason)
+ - [ ] If you've fixed a bug or added code that should be tested, add tests!
+ - [ ] Documentation in `docs` is updated
+ - [ ] `CHANGELOG.md` is updated
+ - [ ] `README.md` is updated
+ - [ ] `LICENSE.md` is updated with new contributors