docs: Add PULL_REQUEST_TEMPLATE.md

Author: erikmd

.github/PULL_REQUEST_TEMPLATE.md

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+* **Kind:** bugfix / enhancement / feature
+
+<!-- For a bug fix, make sure the bug was already reported in an issue. -->
+
+* Close #…
+
+### Description
+
+<!-- Please recap the motivation and/or implementation choices for this change. -->
+
+### Checklist
+
+<!-- You can remove all the check-boxes that are not applicable. -->
+
+* [ ] Read the [CONTRIBUTING.md](https://github.com/ocaml-sf/learn-ocaml/blob/master/CONTRIBUTING.md) guide.
+* [ ] Add/update [tests](https://github.com/ocaml-sf/learn-ocaml/tree/master/tests#readme)
+  <!-- if the change impacts the grading feature. -->
+* [ ] Add/update [documentation](https://github.com/ocaml-sf/learn-ocaml/tree/master/docs)
+  <!-- if there are some user-facing changes. -->
+* [ ] …
+  <!-- you can add more items to summarize what remains to do. -->
+
+<!-- You can leave this note below as a reminder for maintainers: -->
+### Note to maintainers
+
+* Read [this wiki page](https://github.com/ocaml-sf/learn-ocaml/wiki/Checklist-for-testing-and-merging-a-PR)
+* Make sure the PR has a milestone
+* Assign yourself before merging