Updated OAuth2 authentication docs.

Since I got stuck with this for 5 minutes I updated OAuth2 authentication docs to explicitly state that "rest_framework.authentication.OAuth2Authentication" should be added to DEFAULT_AUTHENTICATION_CLASSES.

I also created a working example of implementing OAuth2 paired with SessionAuthentication (for browseable API) in case there's interest:
https://github.com/erkarl/django-rest-framework-oauth2-provider-example