add some note to the PR template

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,18 +1,24 @@
 <!-- Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. -->
 
 **What kind of change does this PR introduce?**
+
 <!-- E.g. a bugfix, feature, refactoring, build related change, etc… -->
 
 **Did you add tests for your changes?**
 
-**If relevant, did you update the documentation?**
+<!-- Note that we won't merge your changes if you don't add tests -->
+
+**If relevant, link to documentation update:**
+
+<!-- Link PR from webpack/webpack.js.org here, or N/A -->
 
 **Summary**
 
 <!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->
 <!-- Try to link to an open issue for more information. -->
 
 **Does this PR introduce a breaking change?**
+
 <!-- If this PR introduces a breaking change, please describe the impact and a migration path for existing applications. -->
 
 **Other information**