Added docs how to migrate existing models to polymorphic

it get's asked a lot.

docs/index.rst

@@ -57,6 +57,7 @@ Advanced topics
 .. toctree::
    :maxdepth: 2
 
+   migrating
    advanced
    managers
    third-party

docs/migrating.rst

@@ -0,0 +1,65 @@
+Migrating existing models to polymorphic
+========================================
+
+Existing models can be migrated to become polymorphic models.
+During the migrating, the ``polymorphic_ctype`` field needs to be filled in.
+
+This can be done in the following steps:
+
+1. Inherit your model from :class:`~polymorphic.models.PolymorphicModel`
+2. Create a Django migration file to create the ``polymorphic_ctype_id`` database column.
+2. Make sure the proper :class:`~django.contrib.contenttypes.models.ContentType` value is filled in.
+
+Filling the content type value
+------------------------------
+
+The following Python code can be used to fill the value of a model:
+
+.. code-block:: python
+
+    from django.contrib.contenttypes.models import ContentType
+    from myapp.models import MyModel
+
+    new_ct = ContentType.objects.get_for_model(MyModel)
+    MyModel.objects.filter(polymorphic_ctype__isnull=True).update(polymorphic_ctype=new_ct)
+
+The creation and update of the ``polymorphic_ctype_id`` column
+can be included in a single Django migration. For example:
+
+.. code-block:: python
+
+    # -*- coding: utf-8 -*-
+    from __future__ import unicode_literals
+    from django.db import migrations, models
+
+
+    def forwards_func(apps, schema_editor):
+        MyModel = apps.get_model('myapp', 'MyModel')
+        ContentType = apps.get_model('contenttypes', 'ContentType')
+
+        new_ct = ContentType.objects.get_for_model(MyModel)
+        MyModel.objects.filter(polymorphic_ctype__isnull=True).update(polymorphic_ctype=new_ct)
+
+
+    def backwards_func(apps, schema_editor):
+        pass
+
+
+    class Migration(migrations.Migration):
+
+        dependencies = [
+            ('contenttypes', '0001_initial'),
+            ('myapp', '0001_initial'),
+        ]
+
+        operations = [
+            migrations.AddField(
+                model_name='mymodel',
+                name='polymorphic_ctype',
+                field=models.ForeignKey(related_name='polymorphic_myapp.mymodel_set+', editable=False, to='contenttypes.ContentType', null=True),
+            ),
+            migrations.RunPython(forwards_func, backwards_func),
+        ]
+
+It's recommended to let ``makemigrations`` create the migration file,
+and include the ``RunPython`` manually before running the migration.