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)
Merge pull request #43 from cf-guardian/nsinit_usage

Improve nsinit usage instructions