add note to upgrade log.

upgrade.md

@@ -11,3 +11,4 @@
 - Edit `app/controllers/BaseController.php` change `use Illuminate\Routing\Controllers\Controller;` to `use Illuminate\Routing\Controller;
 `
 - If you are overriding `missingMethod` in your controllers, add $method as the first parameter.
+- If you are registering model observers in a "start" file, move them to the `App::booted` handler.