Make get.docker.io install directions slightly better by telling user exactly how to add themselves to the "docker" group

As a bonus, if the user has run this script the way we recommend (ie, without "sudo" or "su", run as their user), we can actually give them the exact command they'll need with their correct username substituted appropriately.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3822 from tianon/install-your-user

Make get.docker.io install directions slightly better by telling user exactly how to add themselves to the "docker" group