Added a missing validator example

Author: javiereguiluz

components/validator/metadata.rst

@@ -70,3 +70,28 @@ Classes
 Some constraints allow to validate the entire object. For example, the
 :doc:`Callback </reference/constraints/Callback>` constraint is a generic
 constraint that's applied to the class itself.
+
+Suppose that the class defines a ``validate()`` method to hold its custom
+validation logic::
+
+        // ...
+        use Symfony\Component\Validator\Context\ExecutionContextInterface;
+
+        public function validate(ExecutionContextInterface $context)
+        {
+            // ...
+        }
+
+Then, add the Validator component configuration to the class::
+
+    // ...
+    use Symfony\Component\Validator\Mapping\ClassMetadata;
+    use Symfony\Component\Validator\Constraints as Assert;
+
+    class Author
+    {
+        public static function loadValidatorMetadata(ClassMetadata $metadata)
+        {
+            $metadata->addConstraint(new Assert\Callback('validate'));
+        }
+    }