README: add contiv to cni supported networking
README: add contiv to cni supported networking

dcbw/cni,containernetworking/plugins,rosenhouse/cni,murat1985/cni,mcastelino/cni,kad/cni,dcbw/cni,squaremo/cni,steveeJ/cni,containernetworking/cni,appc/cni,luxas/cni,mcastelino/cni,squaremo/cni,appc/cni,murat1985/cni,squeed/plugins,squeed/plugins,steveeJ/cni,containernetworking/cni,containernetworking/plugins,tomdee/cni,achanda/cni,kad/cni,luxas/cni,tangle329/cni,achanda/cni,tomdee/cni,rosenhouse/cni,tangle329/cni