Clarify how to generate signed urls (#100)

docs/get-started/routing.mdx

@@ -70,16 +70,20 @@ Select provider:
 Based on your answers, it will generate a table with the following columns:
 
 ```shell
-+-------------+--------------------------------------------------------------------------------+
-| Provider    | URL                                                                            |
-+-------------+--------------------------------------------------------------------------------+
-| App Store   | http://localhost/liap/notifications?signature=<signature>&provider=app-store   |
-| Google Play | http://localhost/liap/notifications?signature=<signature>&provider=google-play |
-+-------------+--------------------------------------------------------------------------------+
++-------------+--------------------------------------------------------------------------------------+
+| Provider    | URL                                                                                  |
++-------------+--------------------------------------------------------------------------------------+
+| App Store   | https://yourdomain.com/liap/notifications?signature=<signature>&provider=app-store   |
+| Google Play | https://yourdomain.com/liap/notifications?signature=<signature>&provider=google-play |
++-------------+--------------------------------------------------------------------------------------+
 ```
 
 The generated URLs will be used to receive the [server notifications](/docs/category/server-notifications).
 
+If the URLs start with `http://localhost`, you should modify your Laravel configuration, typically by setting the
+APP_URL environment variable to your desired domain, and rerun `php artisan liap:url`. Do _not_ manually edit the
+URLs, as that will invalidate the signature.
+
 ## Validating a signed route requests
 
 In order to verify that an incoming request has a valid signature, you should set the `routing.signed` key to `true`.