docs: add instruction to change route

Author: kenjis

docs/customization/extending_controllers.md

@@ -1,5 +1,7 @@
 # Extending the Controllers
 
+## Provided Controllers
+
 Shield has the following controllers that can be extended to handle
 various parts of the authentication process:
 
@@ -9,6 +11,8 @@ various parts of the authentication process:
 -   **MagicLinkController** handles the "lost password" process that allows a user to login with a link sent to their email. This allows you to
     override the message that is displayed to a user to describe what is happening, if you'd like to provide more information than simply swapping out the view used.
 
+## How to Extend
+
 It is not recommended to copy the entire controller into **app/Controllers** and change its namespace. Instead, you should create a new controller that extends
 the existing controller and then only override the methods needed. This allows the other methods to stay up to date with any security
 updates that might happen in the controllers.
@@ -29,3 +33,5 @@ class LoginController extends ShieldLogin
     }
 }
 ```
+
+After extending, don't forget to change the route. See [Customizing Routes](./route_config.md).