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)
No problem.  Plus if you use something like getting the hosts network interfaces in the container you actually want it to affect the host system.  Just a different usecase that some ppl need