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>
FWIW I believe this is because e.g. OS X doesn't have wget installed by default, so curl tends to be a little more portable.