Add assumptions from the grootfs/garden integration to README
Add assumptions from the grootfs/garden integration to README

[finishes #133509973]

Signed-off-by: Tiago Scolari <f438736fd7dd1cb07f8d2a37f4499d0a0272a752@pivotal.io>

masters-of-cats/grootfs-release,masters-of-cats/grootfs-release