Add pull request template

.github/pull_request_template.md

@@ -0,0 +1,6 @@
+## Checklist
+
+When making changes to schema files under the [yml](yml) directory:
+
+- [ ] Incremented the schema version number, following the [guidelines](CONTRIBUTING.md#typical-schema-changes) in the [contribution guide](CONTRIBUTING.md)
+- [ ] Referred to the [documentation on specific schemas](CONTRIBUTING.md#specific-schema-documentation) for additional versioning information, change constraints, or tasks that may need to be performed, based on which schema is being updated