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
tar: fixing extracting files that are dirs https://github.com/coreos/rocket/issues/250
tar: ensuring dir mode extraction
tar: fixing extracting files that are dirs #251

Merge pull request #251 from mcuadros:master
docs: update getting started guides
Merge pull request #274 from jonboulle/fixguide

docs: update getting started guides
Create parent directories for every file type and not for only regular files.
Merge pull request #275 from sgotti/tarcreateparenteveryfile

pkg/tar: create parent directories for every file type
cas: clean blockTransform, panic intentionally

There was a bug in diskv (https://github.com/peterbourgon/diskv/pull/20)
which meant that the blockTransform function could be passed an empty
string. In our case this just paniced after trying to split + access
parts of the resulting empty slice.

Panicking is arguably correct behaviour (if we ever get something that's
not a hash it's likely a bug in our code), but let's be more intentional
about it.
Merge pull request #277 from jonboulle/casprefix

cas: clean blockTransform, panic intentionally
Godeps: bump peterbourgon/diskv dependency

This will allow us to take advantage of several new features like
`Import` (eliminating a copy from cas) and a "no cache" mode (to prevent
OOMs), as well as various bug fixes.
Merge pull request #278 from jonboulle/master

Godeps: bump peterbourgon/diskv dependency
cas: never cache values in memory
Merge pull request #279 from jonboulle/master

cas: never cache values in memory
cas: use tmpfile when writing into disk store

Instead of writing into a temporary disk store, calculating the hash,
and then rewriting into a new entry in the diskstore with the hash as
the key, write to a tempfile on disk outside the store and then use the
new Import method to move the file into the store.