Add note about driver API breakage
Add note about driver API breakage

This adds a note about the breakage we caused to our driver API behavior
during Liberty, and resolutions that out of tree drivers may use to fix
themselves.

See also:
http://lists.openstack.org/pipermail/openstack-dev/2015-October/076205.html

Change-Id: I2671e7c8634f3fd3115697df6f0cab68c589d21d
Closes-Bug: #1502980

dims/ironic,bacaldwell/ironic,openstack/ironic,ionutbalutoiu/ironic,NaohiroTamura/ironic,ionutbalutoiu/ironic,dims/ironic,hpproliant/ironic,bacaldwell/ironic,SauloAislan/ironic,openstack/ironic,pshchelo/ironic,pshchelo/ironic,SauloAislan/ironic,NaohiroTamura/ironic,devananda/ironic