Add root device hints documentation
Add root device hints documentation

This patch is documenting how to configure the node to pick the right
device at deployment time using root device hints.

Implements: blueprint root-device-hints
Change-Id: I6cb644ceefcb48caf35fab21141f1b14eaacf724

hpproliant/ironic,Tan0/ironic,naototty/vagrant-lxc-ironic,pshchelo/ironic,SauloAislan/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,openstack/ironic,rdo-management/ironic,SauloAislan/ironic,dims/ironic,NaohiroTamura/ironic,ionutbalutoiu/ironic,naterh/ironic,supermari0/ironic,pshchelo/ironic,redhat-openstack/ironic,naototty/vagrant-lxc-ironic,bacaldwell/ironic,devananda/ironic,dims/ironic,openstack/ironic,NaohiroTamura/ironic