Add documentation for override Swagger docs (#236)

Author: Timherlaud

core/swagger.md

@@ -0,0 +1,64 @@
+# Swagger Support
+
+## Override Swagger documentation
+Symfony allows to [decorate services](https://symfony.com/doc/current/service_container/service_decoration.html), here we need to decorate 
+`api_platform.swagger.normalizer.documentation`
+### Example
+In the following example, we will see how to override the title of the Swagger documentation and add a custom filter for the `GET` operation of `/foos` path 
+
+```yaml
+ # app/config/services.yml
+ 
+services:
+  app.swagger.swagger_decorator:
+     decorates: api_platform.swagger.normalizer.documentation
+     class: 'AppBundle\Swagger\SwaggerDecorator'
+     arguments: ['@app.swagger.swagger_decorator.inner']
+```
+
+```php
+<?php
+
+// src\AppBundle\Swagger\SwaggerDecorator.php
+
+namespace AppBundle\Swagger;
+
+use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
+
+final class SwaggerDecorator implements NormalizerInterface
+{
+    private $decorated;
+
+    public function __construct(NormalizerInterface $decorated)
+    {
+        $this->decorated = $decorated;
+    }
+
+    public function normalize($object, $format = null, array $context = [])
+    {
+        $docs = $this->decorated->normalize($object, $format, $context);
+
+        $customDefinition = [
+            'name' => 'fields',
+            'definition' => 'Fields to remove of the outpout',
+            'default' => 'id',
+            'in' => 'query',
+        ];
+
+		
+	// e.g. add a custom parameter 
+		$docs['paths']['/foos']['get']['parameters'][] = $customDefinition;
+		
+		// Override title
+		$docs['info']['title'] = 'My Api Foo';
+			
+        return $docs;
+    }
+
+    public function supportsNormalization($data, $format = null)
+    {
+        return $this->decorated->supportsNormalization($data, $format);
+    }
+}
+```
+

index.md

@@ -85,6 +85,7 @@
 23. [AngularJS Integration](core/angularjs-integration.md)
     1.  [Restangular](core/angularjs-integration.md#restangular)
     2.  [ng-admin](core/angularjs-integration.md#ng-admin)
+24. [Swagger Support](core/swagger.md)
 
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