Add pull request template

.github/pull_request_template.md

@@ -0,0 +1,6 @@
+## Checklist
+
+When making changes to YAML files in the [schemas](../python/lsst/sdm_schemas/schemas) directory:
+
+- [ ] If applicable, incremented the schema version number, following the guidelines 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