Fixed django#19120 - Added an example of using ModelAdmin methods for read-only fields.

Thanks Daniele Procida for the patch.

Author: timgraham

docs/ref/contrib/admin/index.txt

@@ -816,15 +816,34 @@ subclass::
 
  By default the admin shows all fields as editable. Any fields in this
  option (which should be a ``list`` or ``tuple``) will display its data
- as-is and non-editable. This option behaves nearly identical to
- :attr:`ModelAdmin.list_display`. Usage is the same, however, when you
- specify :attr:`ModelAdmin.fields` or :attr:`ModelAdmin.fieldsets` the
- read-only fields must be present to be shown (they are ignored otherwise).
+ as-is and non-editable. Note that when specifying :attr:`ModelAdmin.fields`
+ or :attr:`ModelAdmin.fieldsets` the read-only fields must be present to be
+ shown (they are ignored otherwise).
 
  If ``readonly_fields`` is used without defining explicit ordering through
  :attr:`ModelAdmin.fields` or :attr:`ModelAdmin.fieldsets` they will be
  added last after all editable fields.
 
+ A read-only field can not only display data from a model's field, it can
+ also display the output of a a model's method or a method of the
+ ``ModelAdmin`` class itself. This is very similar to the way
+ :attr:`ModelAdmin.list_display` behaves. This provides an easy way to use
+ the admin interface to provide feedback on the status of the objects being
+ edited, for example::
+
+ class PersonAdmin(ModelAdmin):
+ readonly_fields = ('address_report',)
+
+ def address_report(self, instance):
+ return ", ".join(instance.get_full_address()) or \
+ "<span class='errors'>I can't determine this address.</span>"
+
+ # short_description functions like a model field's verbose_name
+ address_report.short_description = "Address"
+ # in this example, we have used HTML tags in the output
+ address_report.allow_tags = True
+
+
 .. attribute:: ModelAdmin.save_as
 
  Set ``save_as`` to enable a "save as" feature on admin change forms.