Updated README to clarify legacy client usage
Updated README to clarify legacy client usage

Also, update it to use code-block - which makes things look much nicer.

Change-Id: I930ab63a5d159cf4cea27b4e2c4d6fd933de04fc

dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,stackforge/python-openstacksdk,dtroyer/os-client-config,openstack/os-client-config,dtroyer/python-openstacksdk,openstack/python-openstacksdk