add a PR template (#71)

Author: keewis

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,7 @@
+<!-- Feel free to remove check-list items aren't relevant to your change -->
+
+- [ ] Closes #xxxx
+- [ ] Tests added
+- [ ] Passes `pre-commit run --all-files`
+- [ ] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
+- [ ] New functions/methods are listed in `api.rst`