Add changelog guidance to PR template

Reviewed By: EdmondChuiHW

Differential Revision: D55419910

fbshipit-source-id: 373916365ceaccaf21febed26b8416f331db0d1b

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,6 +4,16 @@
 
 <!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->
 
+<!--
+Changelog entries should be prefixed with one of the following scopes:
+[Breaking, Feature, Fix, Performance, Deprecated, Experimental, Internal]
+
+Examples:
+  Changelog: [Fix] Respond with HTTP 404 when a bundle entry point cannot be resolved
+  Changelog: [Internal]
+-->
+Changelog:
+
 ## Test plan
 
 <!-- Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI. -->