Add logging section of user guide

[Brian Curtin]

After abandoning https://review.openstack.org/#/c/153127/, we'll need to document the steps to take advantage of the logging messages we have in place

Example:
import logging
logging.basicConfig(filename='python-openstacksdk.log',level=logging.DEBUG, format='%(asctime)s %(levelname)s: %(name)s %(message)s')

[Brian Curtin (brian.curtin)]

We did end up adding openstack.utils.enable_logging, and still need to document its use.