API v1 Doc

This is a fairly straight conversion from the KDS extension doc that was
written up for keystone.

Initial document: https://git.openstack.org/cgit/openstack/identity-api/tree/openstack-identity-api/v3/src/markdown/identity-api-v3-os-kds-ext.md

Change-Id: I4a02ed4c376aae97946a7762b558d51745571ec9