contributing: add pull request template

Added pull_request_template.md, which also includes acceptance of
https://github.com/aws/mit-0.

Author: raduweiss

.github/pull_request_template.md

@@ -0,0 +1,23 @@
+## Reason for this PR
+
+`[Author TODO: add issue # or explain reasoning.]`
+
+## Description of changes
+
+`[Author TODO: add description of changes.]`
+
+## License acceptance
+
+By submitting this pull request, I confirm that my contribution is made under
+the terms of the MIT-0 [1] license.
+
+[1] https://github.com/aws/mit-0
+
+## PR checklist
+
+`[Author TODO: Meet these criteria & check the boxes.]`
+`[Reviewer TODO: Verify checked boxes. Request changes if criteria not met]`
+
+- [ ] The reason for this PR is clearly provided (issue no. or explanation).
+- [ ] The description of changes is clear and encompassing.
+- [ ] All commits in this PR are signed (`git commit -s`).