Update README.md (#7784)
Update README.md (#7784)

Update README for control_plane's external volume type variable
Atoms/kubespray,Atoms/kubespray,kubernetes-sigs/kubespray,Atoms/kubespray,kubernetes-incubator/kubespray,kubernetes-incubator/kubespray,kubernetes-incubator/kargo,kubernetes-sigs/kubespray,kubernetes-incubator/kargo,Atoms/kubespray,kubernetes-sigs/kubespray,kubernetes-sigs/kubespray