Update README.md: added sudo usage

Since docker 0.5.2 sudo is required:
http://blog.docker.io/2013/08/containers-docker-how-secure-are-they/#specific-attack-surface-of-the-docker-daemon

> now uses a UNIX socket instead of a TCP socket bound on 127.0.0.1
Merge pull request #4 from ekalinin/patch-1

Update README.md: added sudo usage