Update README.md to specify NFS version is v4
Update README.md to specify NFS version is v4

Attempting to use longhorn with NFSv3 for backuip causes errors related to rpc.statd, which is not managed by longhorn processes.  Longhorn works properly with NFSv4 servers only.

Signed-off-by: Douglas Mayle <88ee136f5916c22b9fbddbfbeff5f0621a3c1503@dmayle.com>

rancher/longhorn