Adding breaking change warning

CHANGELOG.md

@@ -1,7 +1,11 @@
 <a name="4.0.0-rc.1"></a>
 # [4.0.0-rc.1](https://github.com/angular/angularfire2/compare/4.0.0-rc.0...v4.0.0-rc.1) (2017-06-02)
 
-* **rc:** Update to Firebase JS SDK 4.0 ([9642f5](https://github.com/angular/angularfire2/commit/9642f589ba73add6d49a5818a1109028f8c7729b)
+### Breaking changes
+
+* **rc:** Update to Firebase JS SDK 4.0 ([9642f5](https://github.com/angular/angularfire2/commit/9642f589ba73add6d49a5818a1109028f8c7729b))
+
+In version 4.0 of the Firebase SDK `onAuthStateChanged` is only fired on sign-in and sign-out, [see the Firebase JS SDK changelog for more information.](https://firebase.google.com/support/release-notes/js#4.0.0). The added `AngularFireAuth.idToken: Observable<firebase.User>` behaves as `authState` used to.
 
 <a name="4.0.0-rc0"></a>
 # [4.0.0-rc0](https://github.com/angular/angularfire2/compare/2.0.0-beta.8...v4.0.0-rc0) (2017-05-02)