title: How to set a Laravel Nova field to display as readonly or protected?
body: <p>Within Laravel Nova (v1.0.3), there are several methods that grant fine-grained control of the visibility of a resource field (canSee, showOnDetail, etc.).  I can't find any methods that control if a field is editable. How can I display a field, but prevent the user from being able to edit it (make it readonly)?</p>
  
  <p>For example, I'd like to display the "Created At" field, but I don't want the user to be able to change it.</p>