Document facade root service resolving breaking change

Author: X-Coder264

upgrade.md

@@ -15,6 +15,14 @@ Update your `laravel/framework` dependency to `5.8.*` in your `composer.json` fi
 
 Of course, don't forget to examine any 3rd party packages consumed by your application and verify you are using the proper version for Laravel 5.8 support.
 
+### Facade
+
+#### Facade root service resolving
+
+**Likelihood Of Impact: Low**
+
+The `getFacadeAccessor` method [can now return only the string value representing the identifier/alias of the service](https://github.com/laravel/framework/pull/25525). If you have a facade in your project or package whose `getFacadeAccessor` method returns an object, you should change it so that a string is returned.
+
 ### Routing
 
 #### The `UrlGenerator` Contract