title: "Disabled" option for choiceField - Django
body: <p>I having trouble with a simple question : 
  How to have some "disabled" field in a dropdown menu generated via a modelForm and choiceFied in the django Framework ?</p>
  
  <p>At the moment, I cannot figure out how to obtain such an output :
  -- Root 1 entry -- (disabled)
  -- Elt 1 -- (not disabled)
  -- Root 2 entry -- (disabled)</p>
  
  <p>Do you have any advice ?</p>
  
  <p>Pierre</p>