Documentation: Added documentation for `cnitool`.
Documentation: Added documentation for `cnitool`.

Added documentation to simplify the usage of `cnitool` and the
settings expected to be used with `cnitool`.

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