Add TODO for Updating API Spec (opensearch-project#13621)

Signed-off-by: Kunal Khatua <kkhatua@amazon.com>
Co-authored-by: Peter Nied <peternied@hotmail.com>
Co-authored-by: Peter Nied <petern@amazon.com>

Author: kkhatua

.github/pull_request_template.md

@@ -17,6 +17,7 @@ Resolves #[Issue number to be closed when this PR is merged]
   - [ ] All tests pass
 - [ ] New functionality has been documented.
   - [ ] New functionality has javadoc added
+- [ ] API changes companion pull request [created](https://github.com/opensearch-project/opensearch-api-specification/blob/main/DEVELOPER_GUIDE.md).
 - [ ] Failing checks are inspected and point to the corresponding known issue(s) (See: [Troubleshooting Failing Builds](../blob/main/CONTRIBUTING.md#troubleshooting-failing-builds))
 - [ ] Commits are signed per the DCO using --signoff
 - [ ] Commit changes are listed out in CHANGELOG.md file (See: [Changelog](../blob/main/CONTRIBUTING.md#changelog))