📝 Update release notes

[skip ci]

docs/en/docs/release-notes.md

@@ -56,6 +56,10 @@ Still, for now, consider this very experimental and potentially changing and bre
 * Middleware per router
 * Other features planned
 
+### Breaking Changes
+
+* ♻️ Refactor internals to preserve `APIRouter` and `APIRoute` instances. PR [#15745](https://github.com/fastapi/fastapi/pull/15745) by [@tiangolo](https://github.com/tiangolo).
+
 ### Docs
 
 * 📝 Update FastAPI Cloud deployment instructions. PR [#15724](https://github.com/fastapi/fastapi/pull/15724) by [@alejsdev](https://github.com/alejsdev).