Add migration info

Author: fsbraun

README.rst

@@ -49,6 +49,26 @@ Documentation
 Please head over to the separate `documentation <https://django-filer.readthedocs.io/en/latest/index.html>`_
 for all the details on how to install, configure and use django-filer.
 
+Upgrading
+=========
+
+Version 3.3
+-----------
+
+django-filer version 3 contains a change in security policy for file uploads.
+**By default, binary file or files of unknown type are not allowed to be uploaded.**
+To allow upload of binary files in your project, add
+
+.. code-block:: python
+
+    FILER_REMOVE_FILE_VALIDATORS = [
+        "application/octet-stream",
+    ]
+
+to your project's settings. Be aware that binary files always are a security risk.
+See the documentation for more information on how to configure file upload validators,
+e.g., running files through a virus checker.
+
 
 .. |pypi| image:: https://badge.fury.io/py/django-filer.svg
     :target: http://badge.fury.io/py/django-filer