title: Making readonly for existing field but allowing to add when new inline is created in django admin
body: <p>I have two models with one of them as inline to other. I have made fields of inline model read only.</p>
  
  <pre><code>class FollowUpInLine(admin.TabularInline):
  model = md.FollowUp
  extra = 0
  can_delete = False
  
  def get_readonly_fields(self, request, obj=None):
      if request.user.is_superuser == False:          
          if obj: # editing an existing object
                  return self.readonly_fields + (
                  'follow_up_date',
                  'status_inquiry',
                  'remarks',
                  'followup_done_by',
                  )
      return self.readonly_fields
  </code></pre>
  
  <p>However this does not allows to add new fields when "Add another" is clicked in inline rather it changes them to label with value "None". 
  How can I make fields inline but add when next inline is to added?</p>