MR template: JSON schema checklist item.

.gitlab/merge_request_templates/Default.md

@@ -26,6 +26,7 @@ guidelines](https://gitlab.com/caosdb/caosdb/-/blob/dev/REVIEW_GUIDELINES.md)
 - [ ] All automated tests pass
 - [ ] Reference related issues
 - [ ] Up-to-date CHANGELOG.md (or not necessary)
+- [ ] Up-to-date JSON schema (or not necessary)
 - [ ] Appropriate user and developer documentation (or not necessary)
   - How do I use the software?  Assume "stupid" users.
   - How do I develop or debug the software?  Assume novice developers.