Add some API definitions to the README.
Add some API definitions to the README.

Change-Id: I549de0d298d030308971c2739bf1ff88ef9363e1

NaohiroTamura/ironic-lib,faizan-barmawer/ironic-lib,faizan-barmawer/elytics,citrix-openstack-build/ironic-lib,openstack/ironic-lib