Create PULL_REQUEST_TEMPLATE.md

PULL_REQUEST_TEMPLATE.md

+## Pull Request Prelude
+Thank you for your contribution :)
+
+Please complete the below steps before filing your PR:
+
+ I have read and understood CONTRIBUTING document.
+ I have read and understood  CODE_OF_CONDUCT document.
+ I have included tests for the changes in my PR. If not, I have included a rationale for why I haven't.
+ I understand my PR may be closed if it becomes obvious I didn't actually perform all of these steps.
+## Why this change is necessary and useful
+[Please explain in detail why the changes in this PR are needed.]