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>
The reasoning was to use the same tool here and the step below, to reduce the number of deps. I can convert them both to curl.