Document extra dependancy when using Google Cloud Storage backend

[driesdesmet]

Nowhere in the docs is mentioned that when using the Google Cloud Storage backend, you actually need to install an extra dependancy: google-cloud-storage

When running, an exception is raised:

django.core.exceptions.ImproperlyConfigured: Could not load Google Cloud Storage bindings.
See https://github.com/GoogleCloudPlatform/gcloud-python

So when surfing to the mentioned url, I thought I had to install the all of google python client, but that install a whole suite of dependancies. google-cloud-storaga is enough it seems.

I believe there is a way to install extra's to a python package, something along the lines of pip install django-storages[google-cloud] but I'm not familiar if this still works or how involved that is.

[browniebroke]

Agreed, and we could do something similar for each back-end.
A starting point is being covered in PR #274