changing the apt-get to use -d for downloading only.
updating the readme to note for now building the image needs
to be done manually, later Jenkins can run it periodically

Change-Id: I3f117041e72a083c7ca480a462c3d783d44b4776
Merge pull request #211 from CiscoSystems/rpothier/change_apt_get_to_download

changing the apt-get to use -d for downloading only.