add mozlog documentation #12

The first few times I saw this library, I thought it was a Django library based on what I read in the docs. It's not, though it does have Django-specific things in it.

One thing that's not Django-specific is the mozlog bits.

Currently there's a stub for the logging bits here:

https://github.com/mozilla-services/python-dockerflow/blob/master/src/dockerflow/logging.py

This issue covers moving the logging bits to a non-Django chapter and fleshing them out.