Add check list in pull request template (#794)

Signed-off-by: Lantao Jin <[email protected]>

.github/pull_request_template.md

@@ -0,0 +1,14 @@
+### Description
+_Describe what this change achieves._
+
+### Related Issues
+_List any issues this PR will resolve, e.g. Resolves [...]._
+
+### Check List
+- [ ] Updated documentation (docs/ppl-lang/README.md)
+- [ ] Implemented unit tests
+- [ ] Implemented tests for combination with other commands
+- [ ] Commits are signed per the DCO using `--signoff`
+
+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/sql/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).