README: fix docker, add rkt
README: fix docker, add rkt

AntonioMeireles/coreos-xhyve,TheNewNormal/corectl,AntonioMeireles/corectl,tt/coreos-xhyve,c4milo/coreos-xhyve,jbergstroem/coreos-xhyve,TheNewNormal/corectl,chancez/coreos-xhyve,myobie/coreos-xhyve,marineam/coreos-xhyve,AntonioMeireles/coreos-xhyve,toliaqat/coreos-xhyve,AntonioMeireles/corectl,coreos/coreos-xhyve,ecnahc515/coreos-xhyve