Update PR template for new changelog stuff

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,11 @@
 <!-- Please read https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md before submitting your pull request -->
 
 <!-- Include a Sign-Off as described in https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md#sign-off -->
+
+<!-- To specify text for the changelog entry (otherwise the PR title will be used):
+Notes:
+
+Changelog entries will also appear in element-desktop. For PRs that *only* affect the desktop version:
+Notes: none
+element-desktop notes: <notes>
+-->