Add note about using Facades in Lumen

Author: jeromegamez

README.md

@@ -54,6 +54,9 @@ return [
 // bootstrap/app.php
 
 $app->register(Kreait\Laravel\Firebase\ServiceProvider::class);
+
+// If you want to use the Facades provided by the package
+$app->withFacades();
 ```
 
 ## Upgrade