docs: add document for block device management
docs: add document for block device management

We support block device management for privileged container only.

Signed-off-by: Yunguo Wei <5c2251e67595c9f992429870b15d5673b251b50e@windriver.com>
Signed-off-by: Bruce Ashfield <284cc737d174a260eec2a2a6324a304f84b1ae0f@windriver.com>

masselstine/meta-overc,OverC/meta-overc,mufeng/meta-overc,OverC/meta-overc,masselstine/meta-overc,jwessel/meta-overc,mufeng/meta-overc,masselstine/meta-overc,masselstine/meta-overc,jwessel/meta-overc,OverC/meta-overc,mufeng/meta-overc,OverC/meta-overc,OverC/meta-overc,jwessel/meta-overc,OverC/meta-overc