Provide helpful assistence to migrate code to new 1.0+ format

This makes sure the changes in 4dc20a0213
don't cause a new range of bug reports, for things like "reverse()" no
longer works.

docs/changelog.rst

@@ -5,6 +5,12 @@ Changes in git
 --------------
 
 * **BACKWARDS INCOMPATIBILITY:** Dropped Django 1.8 support.
+* **BACKWARDS INCOMPATIBILITY:** Removed old deprecated code from 1.0, thus:
+
+ * Import managers from ``polymorphic.managers`` (plural), not ``polymorphic.manager``.
+ * Register child models to the admin as well using ``@admin.register()`` or ``admin.site.register()``,
+   as this is no longer done automatically.
+
 * Added ``PolymorphicTypeUndefined`` exception for incomplete imported models.
   When a data migration or import creates an polymorphic model,
   the ``polymorphic_ctype_id`` field should be filled in manually too.