Add pull request template (#687)

.github/pull_request_template.md

@@ -0,0 +1,24 @@
+## Summary
+
+- 
+
+## Why
+
+- 
+
+## Testing
+
+- 
+
+## Backward Compatibility / Risk
+
+- 
+
+## Related Issue
+
+- Closes #
+
+## Checklist
+
+- [ ] Tests added or updated if behavior changed
+- [ ] Docs updated if config or user-facing behavior changed