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>
Merge pull request #220 from penberg/docs/build-gpg

Documentation: Add 'gpg' to build pre-requisites