title: Django Admin: readonly_fields not showing on admin. interface
body: <p>This should be a pretty straight-forward thing, but it's not working as it should.</p>
  
  <p><strong>model</strong></p>
  
  <pre><code>class Lesson(models.Model):
      lesson_name = models.CharField(max_length=150, primary_key=True)
      json_name = models.CharField(max_length=150, default="", null=True)
      activity_type = models.CharField(max_length=20)
      learning_language = models.CharField(max_length=50)
      known_language = models.CharField(max_length=50)
  </code></pre>
  
  <p><strong>admin</strong></p>
  
  <pre><code>from .models import Lesson
  
  class LessonAdmin(admin.ModelAdmin):
      fields = ['lesson_name']
      readonly_fields = ['json_name', 'activity_type', 'learning_language', 'known_language']
  
  admin.site.register(Lesson, LessonAdmin) 
  </code></pre>
  
  <p>But, when I log into the admin site, the fields specified in <code>readonly_fields</code> don't show up at all. I've also tried including the field names in a tuple instead of a list (as the documentation specifies), but that gives me an error even. </p>