Adding readonly wysiwyg field to entity form
You can try the Markup module.  Should do exactly what you're after.


  This module adds the ability for site admins to add "Markup" widgets
  to the form. These essentially let designers of cck content-types
  insert additional markup into the node/edit form to display to
  content-authors. This is the equivalent of adding additional elements
  to $form of the type '#markup'.