update packer README: mkdir $TMPDIR before running packer
update packer README: mkdir $TMPDIR before running packer

Signed-off-by: Alex Ley <2a41ee6808353e326aa27bd06454adf5fa8981ab@pivotal.io>

cloudfoundry-incubator/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,shashidharatd/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,shashidharatd/garden-linux