Update PULL_REQUEST_TEMPLATE to include an API spec change in the checklist. (#162) (#178)

(cherry picked from commit 9a7b217)

Signed-off-by: dblock <dblock@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,16 @@
+### Description
+[Describe what this change achieves]
+
+### Related Issues
+Resolves #[Issue number to be closed when this PR is merged]
+<!-- List any other related issues here -->
+
+### 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/custom-codecs/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).