add pull_request_template

Signed-off-by: Didar Tursunov <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+### Description
+
+_Describe what this change achieves._
+
+### Issues Resolved
+
+_List any issues this PR will resolve, e.g. Closes [...]._
+
+### Check List
+
+- [ ] New functionality includes testing.
+  - [ ] All tests pass
+- [ ] Linter check was successfull - `yarn run lint` doesn't show any errors
+- [ ] Commits are signed per the DCO using --signoff
+- [ ] Changelog was updated.
+
+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/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).