[Validator] Fixed example in README file (fixes #4088)

src/Symfony/Component/Validator/README.md

@@ -8,6 +8,7 @@ annotations, which can then be checked against instances of these classes.
     use Symfony\Component\Validator\Validator;
     use Symfony\Component\Validator\Mapping\ClassMetadataFactory;
     use Symfony\Component\Validator\Mapping\Loader\StaticMethodLoader;
+    use Symfony\Component\Validator\Constraints as Assert;
     use Symfony\Component\Validator\ConstraintValidatorFactory;
 
     $validator = new Validator(