docs(): Add notice about breaking change for custom plugins (#985)

This adds a breaking change notice to our changelog regarding custom plugins:

The frontend bundle will be loaded asynchronously. This is a breaking change that can affect custom plugins that access certain globals in the django template. Please see https://forum.sentry.io/t/breaking-frontend-changes-for-custom-plugins/14184 for more information

Author: billyvg

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+- BREAKING CHANGE: The frontend bundle will be loaded asynchronously (via [#25744](https://github.com/getsentry/sentry/pull/25744)). This is a breaking change that can affect custom plugins that access certain globals in the django template. Please see https://forum.sentry.io/t/breaking-frontend-changes-for-custom-plugins/14184 for more information.
+
 ## 21.6.1
 
 - No documented changes.