General cleanup of README.

- Added CWD context to commands for clarity
- Added note about libseccomp dependency
- Re-worded some English for clarity
- Re-ordered Launching an ACI section for clarity

Signed-off-by: Daniel Farrell <dfarrell@redhat.com>
README: update for v0.1.1 of appc/spec
Merge pull request #272 from jonboulle/v011

README: cleanup and update to spec