chore: add a PR template (#149)

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+## Summary
+A short description of the changes with a link to an open issue.
+
+## Checklist
+Delete items not relevant to your PR:
+- [ ] Unit and integration tests covering the common scenarios were added
+- [ ] A human-readable description of the changes was provided so that we can include it in CHANGELOG later
+- [ ] For significant changes, documentation in README and https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials