doc: add notable-change label mention to PR template

PR-URL: nodejs#49188
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,6 +10,9 @@ For code changes:
 2. Update documentation if relevant.
 3. Ensure that `make -j4 test` (UNIX), or `vcbuild test` (Windows) passes.
 
+If you believe this PR should be highlighted in the Node.js CHANGELOG
+please add the `notable-change` label.
+
 Developer's Certificate of Origin 1.1
 
 By making a contribution to this project, I certify that: