Add note about mentioning bugnum in PR description (oppia#3156)

PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,7 @@
-IMPORTANT NOTE (please read, then delete):
+IMPORTANT NOTES (please read, then delete):
 
-The PR title should start with "Fix #bugnum: " (if applicable), followed by a clear one-line present-tense summary of the changes introduced in the PR. For example: "Fix #bugnum: introduce the first version of the collection editor.".
+* The PR title should start with "Fix #bugnum: " (if applicable), followed by a clear one-line present-tense summary of the changes introduced in the PR. For example: "Fix #bugnum: introduce the first version of the collection editor.".
+
+* Please make sure to mention "#bugnum" somewhere in the description of the PR. This enables Github to link the PR to the corresponding bug.
 
 Please also make sure to follow the [style rules](https://github.com/oppia/oppia/blob/develop/CONTRIBUTING.md#style-rules).