Update readme with instructions about rvm & rbenv
Update readme with instructions about rvm & rbenv

nisanth074/invoker,code-mancers/invoker,nisanth074/invoker,chiliburger/invoker,diskshima/invoker,flanker/invoker,code-mancers/invoker,chiliburger/invoker,code-mancers/invoker,diskshima/invoker,chiliburger/invoker,diskshima/invoker,flanker/invoker,flanker/invoker,nisanth074/invoker