Add detailed guide for v3 API

[Xeite]

Hello. Thanks for good plugin.

Recently, we changed keystone API from v2 to v3, and it makes kitchen does not work correctly.
The reason is somewhat subtle, so I spent a time to find out why.

Keystone v3 API forces tokens explicitly represent user+project or user+domain pairs. So 'openstack_domain_name' is required to use v3 API. If README mentions about it, I think people can use v3 API more easily. :)

[jjasghar]

If you have a PR to make it more obvious I’d love to merge it to help people using V3 of the API.