Clarified default behavior when fields and fieldsets are not set on ModelAdmin.

docs/ref/contrib/admin/index.txt

@@ -337,7 +337,8 @@ subclass::
     If neither ``fields`` nor :attr:`~ModelAdmin.fieldsets` options are present,
     Django will default to displaying each field that isn't an ``AutoField`` and
     has ``editable=True``, in a single fieldset, in the same order as the fields
-    are defined in the model.
+    are defined in the model, followed by any fields defined in
+    :attr:`~ModelAdmin.readonly_fields`.
 
 .. attribute:: ModelAdmin.fieldsets