Add note for GrootFS and stacks
Add note for GrootFS and stacks

GrootFS requires Cloud Foundry stacks to be provided as a path to a tarball rather than a path to a directory.
cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release