Add GitHub pull request template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,8 @@
+Before submitting this pull request, please first:
+
+- [ ] State that it "Resolves #xyz".
+- [ ] Ensure your code is up-to-date with `master`.
+- [ ] Add tests that fail without your changes.
+- [ ] Update all relevant [documentation](../docs).
+- [ ] Assign the PR to the appropriate subsystem [maintainer](https://aka.ms/psowners).
+- [ ] Check that your commits follow our [contributing guidelines](CONTRIBUTING.md)