Update documentation

[joeyworld]

In Usage paragraph, there exists:

In your urls.py add the following URL route to enable obtaining a token via a POST included the user's username and password.

from rest_framework_jwt.views import obtain_jwt_token
#...

urlpatterns = [
    '',
    # ...

    url(r'^api-token-auth/', obtain_jwt_token),
]

I think fixing urlpatterns in django 2.0 way, using path or re-path would be better.

[joeyworld]

If compatible with 2.0, I can work on with it and send a PR