README: add some material on what CNI isn't
README: add some material on what CNI isn't

We get some questions about this stuff from time to time and I want to
acknowledge we have thought about them but aren't currently tackling
them.

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