Add documentation for using a custom member model

Author: bryannielsen

docs/advanced/authentication.md

@@ -1,6 +1,6 @@
 # Authentication
 
-Coilpack comes with it's own Authentication Guard for Laravel so that you can retrieve the active ExpressionEngine member as an Eloquent Model with the following code:
+Coilpack comes with it's own Authentication Guard for Laravel so that you can retrieve the active ExpressionEngine Member as an Eloquent model with the following code:
 
 ```php
 auth('coilpack')->user()
@@ -21,4 +21,36 @@ We are not making any assumptions about your setup and so the `'coilpack'` guard
 
 This can simplify your authentication calls to look like the standard `auth()->user()`.
 
-For more information on [Laravel's Authentication](https://laravel.com/docs/9.x/authentication) capabilities please review their documentation.
+For more information on [Laravel's Authentication](https://laravel.com/docs/9.x/authentication) capabilities please review their documentation.
+
+## Custom Member Model
+
+The Eloquent model used by Coilpack's authentication guard can be extended to add functionality and other customizations.
+
+Instead of starting with a blank Eloquent model we recommend that you extend the included Member model `Expressionengine\Coilpack\Models\Member\Member`
+
+Your custom model might look like this:
+```php
+# app/Models/Member.php
+<?php
+
+namespace App\Models;
+
+use Expressionengine\Coilpack\Models\Member\Member as BaseMember;
+
+class Member extends BaseMember
+{
+
+}
+```
+
+Coilpack doesn't know about this custom model until you update the `member_model` setting in your `config/coilpack.php`.
+
+```php
+    # config/coilpack.php
+    return [
+    ...
+    'member_model' => \App\Models\Member::class,
+    ...
+    ];
+```