README: add some more details on what CNI is
README: add some more details on what CNI is

We needed a bit more intro material to scope what CNI does. Here is my
attempt.

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