Update docs with concept of hosting providers

As a user this is something I would have found *very* useful the first time I read about docker.  The documentation current says the only way to have private repos is to setup your own registry.  It took me several weeks of using docker before I found out there are hosting providers in the community.  Seems like a good idea to point this out to both help new users and to point people to some of the companies supporting the docker community.

Note: I debated putting the links to the companies, but I think it gives user a better experience so they can go directly to a couple of them and it seems inline with how the installation section shows how to install on EC2, Rackspace, Google, etc.