Make admin form field read only in magento

     $fieldset->addField(
            'additional_description',
            'text',
            array(
                'label' => Mage::helper('custom_favourites')->__('Additional Description'),
                'name'  => 'additional_description',
            'required'  => false,
            'readonly' => true,
            'class' => 'required-entry',
           )
        );