add note about fixing old migrations that break on the expired column

docs/releases/0.4.rst

@@ -144,6 +144,16 @@ If you are using the elasticsearch backend, you must install the ``elasticsearch
     If you are using an older version of Elasticsearch (< 1.0) you must install ``elasticsearch`` version 0.4.x.
 
 
+Addition of ``expired`` column may break old data migrations involving pages
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The scheduled publishing mechanism adds an ``expired`` field to wagtailcore.Page, defaulting to False. Any application code working with Page objects should be unaffected, but any code that creates page records using direct SQL, or within existing South migrations using South's frozen ORM, will fail as this code will be unaware of the ``expired`` database column. To fix a South migration that fails in this way, add the following line to the ``'wagtailcore.page'`` entry at the bottom of the migration file:
+
+.. code-block:: python
+
+  'expired': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+
+
 Deprecated features
 ===================