update README.md
update README.md

If user start ryu-manager with `ryu-manager dl_plugin`, it might be
terminate immediately, add `ryu.controller.ofp_handler` to avoid this
situation.

pichuang/Dragon-Knight,Ryu-Dragon-Knight/Dragon-Knight,TakeshiTseng/ryu-dynamic-loader,TakeshiTseng/Dragon-Knight