Clarify in Readme.md what happens if you log out from shibooleth

[KonstantinSchubert]

If I understand the code correctly, logging our from shibboleth means that this comparison in the middleware turns our false:

https://github.com/KonstantinSchubert/django-shibboleth-adapter/blob/master/shibboleth/middleware.py#L38

Thus, logging our from shibboleth means that the user will also be removed from our app.