Update README to mention Vagrant

This adds a link to the boot2docker Vagrant box repo. Alternatively, it'd be cool if you wanted to just take that repo and put it into the boot2docker repo to make it easier to discover. It barely modifies boot2docker, just adding the Vagrant SSH key to it. 

In the next version of Vagrant, this won't even be necessary since Vagrant will be able to auth with SSH passwords. 

Related to #29