add info about using linked clones for fast dev deployments

[Vinnie Mazza]

• readme needs info on creating snapshot
• variables need the state mapped.

variable "vm_linked_clones" {
  description = <<EOF
  "use linked clones for development, set to false for non development deployments.
  Note: disk size must match the source template exactly in this mode"
  EOF
  default     = false
}