stage1: gitignore stage1/cache
Documentation: Add 'gpg' to build pre-requisites

Commit 00294ed ("stage1: gpg verify and cache retrieved coreos image")
added a dependency to 'gpg'. Add that to list of build pre-requisites.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
stage1: add / to stage1/cache gitignore entry
Merge pull request #220 from penberg/docs/build-gpg

Documentation: Add 'gpg' to build pre-requisites
Merge pull request #216 from jzelinskie/ignore-cache

stage1: gitignore stage1/cache
Documentation: Adding version to hello world example

The actool is failing when validating the manifest.json, as version is missing.
stage1: generate a socket unit for socket-activated ports

 * Generate a systemd socket unit for socket-activated ports
 * Add socket-related path generators and cleanup rest
 * Remove unnecessary systemd unit path mkdirs, just trust the
   stage1 rootfs provides them.
Merge pull request #214 from vcaputo/socketactivation

stage1: generate socket unit for socket-activated ports