Add configuration file override

Author: josbeir

README.md

@@ -127,25 +127,40 @@ The hub will be available at `http://localhost:3000/.well-known/mercure`.
 
 ## Configuration
 
-The plugin includes a default configuration file with all available options. The configuration is automatically loaded from the plugin's `config/mercure.php` file.
+The plugin comes with sensible defaults and multiple configuration options.
 
-Set the required environment variables in your `.env` file:
+**Quick Setup (Environment Variables):**
+
+For development, the fastest way to get started is using environment variables in your `.env` file:
 
 ```env
 MERCURE_URL=http://localhost:3000/.well-known/mercure
 MERCURE_PUBLIC_URL=http://localhost:3000/.well-known/mercure
 MERCURE_JWT_SECRET=!ChangeThisMercureHubJWTSecretKey!
 ```
 
+**Configuration Files:**
+
+The plugin loads configuration in this order:
+
+1. **Plugin defaults** - `vendor/josbeir/cakephp-mercure/config/mercure.php` (loaded automatically)
+2. **Your overrides** - `config/app_mercure.php` (optional, loaded after plugin defaults)
+
+Create `config/app_mercure.php` in your project to customize any settings. Your values will override the plugin defaults.
+
+**Cross-Subdomain Setup:**
+
 > [!NOTE]
-> If your Mercure hub is running on a different subdomain than your CakePHP application, you need to set the cookie domain to the top-level domain:
+> If your Mercure hub runs on a different subdomain than your CakePHP application (e.g., `hub.example.com` vs `app.example.com`), you must configure the cookie domain:
 >
 > ```env
-> # For cross-subdomain authorization
+> # Allow cookie sharing across subdomains
 > MERCURE_COOKIE_DOMAIN=.example.com
 > ```
 >
-> This allows the authorization cookie to be accessible by both your application and the Mercure hub when they are on different subdomains of the same parent domain.
+> This enables the authorization cookie to be accessible by both your application and the Mercure hub. Without this setting, authorization will fail for cross-subdomain requests.
+
+For a complete list of available environment variables, see the plugin's `config/mercure.php` file.
 
 **Configuration structure:**
 
@@ -397,7 +412,7 @@ Publisher::publish($update);
 > **View Class Configuration:** By default, `ViewUpdate` uses CakePHP's automatic view class selection (your `AppView` if it exists, otherwise the base `View` class). You can override this by setting `view_class` in your configuration:
 >
 > ```php
-> // In config/mercure.php
+> // In config/app_mercure.php
 > return [
 >     'Mercure' => [
 >         'view_class' => \App\View\CustomView::class,

src/MercurePlugin.php

@@ -26,6 +26,11 @@ public function bootstrap(PluginApplicationInterface $app): void
 
         // Load plugin configuration
         Configure::load('Mercure.mercure');
+
+        // Load app specific config file.
+        if (file_exists(ROOT . DS . 'config' . DS . 'app_mercure.php')) {
+            Configure::load('app_mercure');
+        }
     }
 
     /**