Added note for $extend() migration notes

Author: Jacob Wenger

changelog.txt

@@ -12,7 +12,7 @@ changed - `$asArray()` has been replaced by calling the `$firebaseArray` service
 changed - `$asObject()` has been replaced by calling the `$firebaseObject` service directly.
 changed - `$FirebaseArray` renamed to $firebaseArray`.
 changed - `$FirebaseObject` renamed to `$firebaseObject`.
-changed - `$extendFactory()` has been renamed to `$extend()`.
+changed - `$extendFactory()` has been renamed to `$extend()` and has been simplified; see the migration guide for details.
 changed - A warning message is now displayed when trying to sync to array-like data, since this is error-prone.
 changed - Upgraded Angular dependency to 1.3.x and 1.4.x (Angular 1.2.x should still work but it is no longer officially supported).
 fixed - `$value` is now removed when data changes from a primitive to an object with child keys (thanks @jamesstalmage).