Add documentation for inheritable tracking

Author: macro1

CHANGES.rst

@@ -1,6 +1,10 @@
 Changes
 =======
 
+tip (unreleased)
+----------------
+- History tracking can be inherited by passing `inherit=True`. (gh-63)
+
 1.7.0 (2015-12-02)
 ------------------
 - Add ability to list history in admin when the object instance is deleted. (gh-72)

docs/advanced.rst

@@ -65,6 +65,35 @@ third-party apps you don't have control over.  Here's an example of using
     register(User)
 
 
+Allow tracking to be inherited
+---------------------------------
+
+By default history tracking is only added for the model that is passed
+to ``register()`` or has the ``HistoricalRecords`` descriptor. By
+passing ``inherit=True`` to either way of registering you can change
+that behavior so that any child model inheriting from it will have
+historical tracking as well. Be careful though, in cases where a model
+can be tracked more than once, ``MultipleRegistrationsError`` will be
+raised.
+
+.. code-block:: python
+
+    from django.contrib.auth.models import User
+    from django.db import models
+    from simple_history import register
+    from simple_history.models import HistoricalRecords
+
+    # register() example
+    register(User, inherit=True)
+
+    # HistoricalRecords example
+    class Poll(models.Model):
+        history = HistoricalRecords(inherit=True)
+
+Both ``User`` and ``Poll`` in the example above will cause any model
+inheriting from them to have historical tracking as well.
+
+
 .. recording_user:
 
 Recording Which User Changed a Model