minor #9724 Add mandatory method and attribute (razbounak)

This PR was squashed before being merged into the 2.7 branch (closes #9724).

Discussion
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Add mandatory method and attribute

For the page "Implement a Simple Registration Form"
Without $roles, construct(), getRoles() and eraseCredentials(), this entity doesn't work because of his interface
It may be nice to add these methods so that you do not have any errors when you copy paste

Commits
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9ddb69d Add mandatory method and attribute

Author: javiereguiluz

doctrine/registration_form.rst

@@ -87,6 +87,15 @@ With some validation added, your class may look something like this::
          * @ORM\Column(type="string", length=64)
          */
         private $password;
+        
+        /**
+         * @ORM\Column(type="array")
+         */
+        private $roles;
+
+        public function __construct() {
+            $this->roles = array('ROLE_USER');
+        }
 
         // other properties and methods
 
@@ -136,8 +145,15 @@ With some validation added, your class may look something like this::
             // You *may* need a real salt if you choose a different encoder.
             return null;
         }
+        
+        public function getRoles()
+        {
+            return $this->roles;
+        }
 
-        // other methods, including security methods like getRoles()
+        public function eraseCredentials()
+        {
+        }
     }
 
 The :class:`Symfony\\Component\\Security\\Core\\User\\UserInterface` requires