Add note to upgrade guide.

upgrade.md

@@ -8,3 +8,4 @@
 - Remove call to `redirectIfTrailingSlash` in `bootstrap/start.php` file.
 - Edit `app/config/app.php`; in `aliases` change `'Controller' => 'Illuminate\Routing\Controllers\Controller',`
   to use `Illuminate\Routing\Controller`
+- If you are overriding missingMethod in your controllers, add $method as the first parameter.