doc: update readme to add ROX support
doc: update readme to add ROX support

Updated Readme to add ROX support
for rbd PVC.

Signed-off-by: Madhu Rajanna <60fbf37a92c2a0908e31296d18dadbec5a4dae17@gmail.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi