add issue requirement to pull request template

.github/pull_request_template.md

@@ -2,8 +2,17 @@
 Thank you for contributing to TiDB! Please read TiDB's [CONTRIBUTING](https://github.com/pingcap/tidb/blob/master/CONTRIBUTING.md) document **BEFORE** filing this PR.
 -->
 
-### What problem does this PR solve? <!--add issue link with summary if exists-->
+### What problem does this PR solve?
+<!--
+Please create an issue first to describe the problem.
+
+There MUST be one line starting with "Issue Number:  " and 
+linking the relevant issues via the "close" or "ref".
+
+For more info, check https://pingcap.github.io/tidb-dev-guide/contribute-to-tidb/contribute-code.html#referring-to-an-issue.
+ -->
 
+Issue Number: close #xxx
 
 ### What is changed and how it works?
 
@@ -35,4 +44,4 @@ Related changes
  - Need to cherry-pick to the release branch
  - Need to update the documentation
  - Need to update the `tidb-ansible` repository
- - Need to be included in the release note
+ - Need to be included in the release note