Update PULL_REQUEST_TEMPLATE to include an API spec change in the checklist. (#1202)

Author: dblock

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,16 @@
-*Issue #, if available:*
+### Description
+[Describe what this change achieves]
 
-*Description of changes:*
+### Related Issues
+Resolves #[Issue number to be closed when this PR is merged]
+<!-- List any other related issues here -->
 
-*CheckList:*
-- [ ] Commits are signed per the DCO using --signoff
+### Check List
+- [ ] New functionality includes testing.
+- [ ] New functionality has been documented.
+- [ ] API changes companion pull request [created](https://github.com/opensearch-project/opensearch-api-specification/blob/main/DEVELOPER_GUIDE.md).
+- [ ] Commits are signed per the DCO using `--signoff`.
+- [ ] Public documentation issue/PR [created](https://github.com/opensearch-project/documentation-website/issues/new/choose).
 
 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
 For more information on following Developer Certificate of Origin and signing off your commits, please check [here](https://github.com/opensearch-project/index-management/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).