Improve nsinit usage instructions

nsinit must be run as root.

Tidy up the README somewhat to clarify the distinction between libcontainer and the nsinit CLI.

Fix some typos in other files.

Docker-DCO-1.1-Signed-off-by: Glyn Normington <gnormington@gopivotal.com> (github: glyn)
Whoops. A Warden assumption crept in there. Will delete this.