Add git and update instructions to README
Add git and update instructions to README

Signed-off-by: Will Pragnell <eefda05bdef438660d447ff971b84a00b7795fad@pivotal.io>

cloudfoundry-incubator/garden-linux-release,cloudfoundry-incubator/garden-linux-release,cloudfoundry-incubator/garden-linux-release,cloudfoundry-incubator/garden-linux-release,cloudfoundry-incubator/garden-linux-release