title: Symfony2 Sonata Admin show attribute only as a readyonly text
body: <p>I have some immutable attributes on my entity to administrate with sonata-admin bundle.</p>
  
  <p>I want to show them in the edit-view of the entity, but don't want to provide any mechanism to change it (e.g. the value shall not be inside a input field)</p>
  
  <p>I couldn't find anything but this:</p>
  
  <pre><code>$formMapper
      -&gt;add('post', 'entity', 
          array(
              'label' =&gt; 'Some post', 
              'attr' =&gt; array(
                  'readonly' =&gt; true,
                  'disabled' =&gt; true
              ),
              'class' =&gt; 'Acme\DemoBundle\Entity\Post'
          )
      )
  ;
  </code></pre>
  
  <p>I tried it out with <code>read_only</code>, <code>readonly</code>, <code>disabled</code> etc. all the stuff. It looks ok, it's now inside a dropdown (since it is an entity) and I can not modify it.</p>
  
  <p>But I even don't want that. I really need it as text (the current one).</p>
  
  <p><em>Especially this is annoying if you use DoctrineExtensions with softdeletable, timestampable, since every "save" saves also the form-data.</em></p>
  
  <p>Changing the type to 'text' instead of 'entity' replaces the dropdown with a input-field.. So, what's the best approach here?</p>