chore: add PR template

Author: Marc Jakobi

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,11 @@
+<!-- markdownlint-disable -->
+###### Description of changes
+
+###### Things done
+
+- [ ] Tested, as applicable:
+  - [ ] Manually
+  - [ ] Added tests
+- [ ] Updated [CHANGELOG.md](https://github.com/mrcjkb/haskell-snippets.nvim/blob/master/CHANGELOG.md)
+  (if applicable).
+- [ ] Fits [CONTRIBUTING.md](https://github.com/mrcjkb/haskell-snippets.nvim/blob/master/CONTRIBUTING.md)