gitlab: Add merge request template

This begins to define our expectations of contributions.

[skip-ci]

.gitlab/merge_request_templates/merge-request.md

+Thank you for your contribution to GHC!
+
+Please take a few moments to verify that your commits fulfill the following:
+
+ * [ ] are either individually buildable or squashed
+ * [ ] have commit messages which describe *what they do*
+   (referring to [Notes][notes] and tickets using `#NNNN` syntax when
+   appropriate)
+ * [ ] have added source comments describing your change. For larger changes you
+   likely should add a [Note][notes] and cross-reference it from the relevant
+   places.
+ * [ ] add a [testcase to the
+   testsuite](https://ghc.haskell.org/trac/ghc/wiki/Building/RunningTests/Adding).
+
+If you have any questions don't hesitate to open your merge request and inquire
+in a comment. If your patch isn't quite done yet please do add prefix your MR
+title with `WIP:`.
+
+[notes]: https://ghc.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Commentsinthesourcecode