Update PR template with a comment about release notes (ChuckerTeam#548)

* Add comment about updating release notes

* Update PULL_REQUEST_TEMPLATE

Add comma to reminder

.github/PULL_REQUEST_TEMPLATE

@@ -6,6 +6,7 @@
 
 ## :pencil: Changes
 <!-- Which code did you change? How? -->
+<!-- If your changes affect users somehow, be it a new feature, a bugfix or an API change please update the `Unreleased` section in the CHANGELOG.md file.
 
 ## :paperclip: Related PR
 <!-- PR that blocks this one, or the ones blocked by this PR -->