Update README.md: added sudo usage

Since [docker 0.5.2](http://blog.docker.io/2013/08/containers-docker-how-secure-are-they/#specific-attack-surface-of-the-docker-daemon) ``sudo`` is required:

> For this reason, the REST API endpoint (used by the Docker CLI to communicate with the Docker daemon) changed in Docker 0.5.2, and now uses a UNIX socket instead of a TCP socket bound on 127.0.0.1 (the latter being prone to cross-site-scripting attacks if you happen to run Docker directly on your local machine, outside of a VM). You can then use traditional UNIX permission checks to limit access to the control socket.