chore: add pull request template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+<!-- Please describe the current behavior and link to a relevant issue. -->
+Fixes <issue-number>
+
+**If your build fails** due to your commit message not passing the build checks, please review the guidelines here: https://github.com/emqx/emqx/blob/master/CONTRIBUTING.md.
+
+## PR Checklist
+Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:
+
+- [ ] Added tests for the changes
+- [ ] Changed lines covered in coverage report
+- [ ] Change log has been added to `changes/` dir
+- [ ] For EMQX 4.x: `appup` files updated (execute `scripts/update-appup.sh emqx`)
+- [ ] For internal contributor: there is a jira ticket to track this change, and another jira tickt to track doc updates (if any)
+- [ ] In case of non-backward compatible changes, reviewer should check this item as a write-off, and add details in **Backward Compatibility** section
+
+## Backward Compatibility
+
+## More information