Updated Packer build example's README (#534)
Updated Packer build example's README (#534)

GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community