title: validation on unbound form field
body: <p>I have a form with an extra unbound field added using 'property_path' => false.</p>
  
  <p>I would like to have a simple validation on this field and I've found many answers that suggest to use something like </p>
  
  <pre><code>$builder-&gt;addValidator(...);
  </code></pre>
  
  <p>but I've seen that in symfony 2.1 $builder->addValidator is deprecated. Does anyone know what is the correct way to put a validation on an unbound field in Symfony 2.1?</p>