Update devicemapper.md
Update devicemapper.md

Add Device Mapper specific command reference.
rancher/convoy,aruneli/convoy,rancherio/volmgr,nickvanw/convoy,rancherio/rancher-volume,medallia/convoy,rancherio/volmgr,rancher/convoy,jinuxstyle/convoy,rancherio/rancher-volume,rancher/rancher-volume,rancher/rancher-volume,rancherio/volmgr,medallia/convoy,aruneli/convoy,nickvanw/convoy,nickvanw/convoy,medallia/convoy,jinuxstyle/convoy,rancherio/rancher-volume,rancher/convoy,jinuxstyle/convoy,rancher/rancher-volume,aruneli/convoy