Added an example

To include an annotation Group for Serializer.

Author: Gary Houbre

serializer.rst

@@ -138,11 +138,53 @@ To use annotations, first add support for them via the SensioFrameworkExtraBundl
     $ composer require sensio/framework-extra-bundle
 
 Next, add the :ref:`@Groups annotations <component-serializer-attributes-groups-annotations>`
-to your class and choose which groups to use when serializing::
+to your class::
+
+    // src/Entity/Product.php
+    namespace App\Entity;
+    
+    use Doctrine\ORM\Mapping as ORM;
+    use Symfony\Component\Serializer\Annotation\Groups;
+    
+    /**
+    * @ORM\Entity(repositoryClass="App\Repository\ProductRepository")
+    */
+    class Product
+    {
+    
+       /**
+         * @ORM\Id
+         * @ORM\GeneratedValue
+         * @ORM\Column(type="integer")
+         * @Groups({"show_product", "list_product"})
+         */
+        private $id;
+
+        /**
+         * @ORM\Column(type="string", length=255)
+         * @Groups({"show_product", "list_product"}) 
+         */
+        private $name;
+
+        /**
+         * @ORM\Column(type="integer")
+         * @Groups({"show_product", "list_product"}) 
+         */
+        private $price;
+        
+        /**
+         * @ORM\Column(type="integer")
+         * @Groups({"show_product"}) 
+         */
+        private $description;
+
+    }
+
+and choose which groups to use when serializing::
 
     $json = $serializer->serialize(
-        $someObject,
-        'json', ['groups' => 'group1']
+        $product,
+        'json', ['groups' => 'show_product']
     );
 
 .. tip::