doc: add v15.4.0 link to CHANGELOG.md
Description
Adding a link to the CHANGELOG.md
for the latest 15.x release.
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Adding a link to the CHANGELOG.md
for the latest 15.x release.