Add a pull-request template

Author: jamesbroadhead

.github/pull_request_template.md

@@ -0,0 +1,7 @@
+### All Submissions:
+
+* [ ] Have you opened an Issue before filing this PR? 
+* [ ] Have you signed our [CLA](https://www.mongodb.com/legal/contributor-agreement)? 
+* [ ] Have you checked to ensure there aren't other open [Pull Requests](../../../pulls) for the same update/change?
+* [ ] Put `closes #XXXX` in your comment to auto-close the issue that your PR fixes (if such).
+