[github]: Update pull request template comments

Author: Shuotian Cheng

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,6 +5,7 @@ https://github.com/Azure/SONiC/blob/gh-pages/CONTRIBUTING.md
 1. Make sure your commit includes a signature generted with `git commit -s`
 2. Make sure your commit title follows the correct format: [component]: description
 3. Make sure your commit message contains enough details about the change and related tests
+4. Make sure your pull request adds related reviewers, asignees, labels
 
 Please also provide the following information in this pull request:
 -->