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>
Do you want these paths removed as well, for copy/paste ease, or is it worth it for the added clarity? Since it's not multi-line, copy/paste usability change seems smaller.