Change single-dash flag usage to double-dash

The single-dash long-form flag usage is deprecated.

Docker-DCO-1.1-Signed-off-by: Nathan LeClaire <nathanleclaire@gmail.com> (github: nathanleclaire)

Docker-DCO-1.1-Signed-off-by: Nathan LeClaire <nathanleclaire@gmail.com> (github: jimperrin)
Updated documentation to include CentOS installation instructions.
Extraneous line removal, description change

Docker-DCO-1.1-Signed-off-by: Jim Perrin <jperrin@centos.org> (github: jimperrin)