readme: add Silk CNI plugin
readme: add Silk CNI plugin

mcastelino/cni,rosenhouse/cni,appc/cni,rosenhouse/cni,dcbw/cni,containernetworking/cni,dcbw/cni,kad/cni,kad/cni,mcastelino/cni,containernetworking/cni,appc/cni