add a link to parent issue

Author: mshustov

.github/pull_request_template.md

@@ -3,6 +3,7 @@
 
 ## Checklist
 Delete items not relevant to your PR:
+- [ ] Closes issue <!-- Link to an issue to close on merge. -->
 - [ ] Unit and integration tests covering the common scenarios were added
 - [ ] A human-readable description of the changes was provided to include in CHANGELOG
 - [ ] For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials