outdated documentation: include urls

[elmcrest]

since django 2.x the urls config has a new way without the need to use regex.

while it's compatible to the old variant, for me the simplest solution to include elephantblog urls was:

from django.urls import path, include
from elephantblog import urls as elephanturls

urlpatterns += [path("blog/", include(elephanturls))]

[matthiask]

I'd still recommend using url() (at least in the docs) as long as we're still supporting Django versions <2.0