README: List cilium as 3rd party plugin
README: List cilium as 3rd party plugin

Signed-off-by: Thomas Graf <17c2d8920621522552b392f72d8fdb4d1e866b44@suug.ch>

mcastelino/cni,containernetworking/plugins,dcbw/cni,dcbw/cni,rosenhouse/cni,appc/cni,appc/cni,rosenhouse/cni,tangle329/cni,containernetworking/cni,squeed/plugins,kad/cni,tangle329/cni,containernetworking/cni,containernetworking/plugins,squeed/plugins,mcastelino/cni,kad/cni