add PULL_REQUEST_TEMPLATE.md

Author: basnijholt

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,7 @@
+<!--
+Thank you for pull request.
+Below are a few things we ask you kindly to self-check before getting a review.
+-->
+Checklist
+* [ ] Check if the plots display on the test documentation [RTD](https://adaptive.readthedocs.io/en/test_documentation/) but make sure it's build first [here](https://readthedocs.org/projects/adaptive/builds/)
+* [ ] Make sure the tests pass