Document usage as django app

[winged]

We should document how to use Emeis as a Django app (and optimize for it as well). It works, but needs some specific adjustments to the settings that should be documented and easy to use.

Quoting from #339:

When integrating this in our project (which is using emeis as a django app), the only thing I stumbled over was that I had to set DEFAULT_FILTER_BACKENDS as well. We don't have docs about usage as a django app yet though, so there is no place to add a hint about this - or am I mistaken?

We don't have any docs yet in this regard - let's do that in a separate task however.

Originally posted by @winged in #339 (comment)