add PULL_REQUEST_TEMPLATE (#688)

Author: davidhewitt

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,17 @@
+<!-- Thank you for your contribution! -->
+
+## Change Summary
+
+<!-- Please give a short summary of the changes. -->
+
+## Related issue number
+
+<!-- Are there any issues opened that will be resolved by merging this change? -->
+<!-- WARNING: please use "fix #123" style references so the issue is closed when this PR is merged. -->
+
+## Checklist
+
+* [ ] Unit tests for the changes exist
+* [ ] Documentation reflects the changes where applicable
+* [ ] Pydantic tests pass with this `pydantic-core` (except for expected changes)
+* [ ] My PR is ready to review, **please add a comment including the phrase "please review" to assign reviewers**