docs: expand PR template with checklist and related issues

Refs opendecree/decree#154

Author: zeevdr

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,19 @@
 ## Summary
 
-What does this PR do?
+
+
+## Related issues
+
+Closes #
 
 ## Test plan
 
 - [ ] `make lint` passes
 - [ ] `make typecheck` passes
-- [ ] `make test` passes (107+ tests, 80%+ coverage)
+- [ ] `make test` passes
+
+## Checklist
+
+- [ ] SDK regenerated if proto changed in `decree`
+- [ ] Breaking change flagged in title/description if applicable
+- [ ] Changelog/release note added if applicable