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)
I would not say that it does not affect the host system.  I should say that it shares the kernel of the host system.