title: Django admin: hide read-only fields on new records?
body: <p>I'm using the Django admin site with some readonly fields on records:</p>
  
  <pre><code>class BookAdmin(admin.ModelAdmin):
      fieldsets = [
      (None, {'fields': ['title', 'library_id', 'is_missing', \
                         'transactions_all_time']}),
      ]
      readonly_fields = ['transactions_all_time',]
      list_display = ('library_id', 'author', 'title')
  </code></pre>
  
  <p>This works great when editing records - the <code>transactions_all_time</code> field is read-only, just as I want.</p>
  
  <p>However, when adding new records it behaves a bit oddly. I get a read-only section at the bottom of the page, which I can't edit and which is irrelevant at this point.</p>
  
  <p>It would be much better if this field was not present at all when adding new records. </p>
  
  <p>Is there any Django option for not displaying read-only fields while adding a new record? I know I can hack the CSS on <code>add_form.html</code> to hide it, but is there a better way?</p>
  
  <p>Thanks. </p>