Update release notes for React on Rails 15.0.0

Added a link to the Changelog for version 15.0.0.

Author: justin808

docs/release-notes/15.0.0.md

@@ -1,5 +1,7 @@
 # React on Rails 15.0.0 Release Notes
 
+Also see the [Changelog for 15.0.0]([url](https://github.com/shakacode/react_on_rails/blob/master/CHANGELOG.md#1500---2025-08-28)).
+
 ## Major Features
 
 ### 🚀 React Server Components Support