image-layout: add explanation of the refs directory use case for tags
image-layout: add explanation of the refs directory use case for tags

Signed-off-by: Brandon Philips <74d2b51265fa19d338eb593a7555ec2b932e59c0@coreos.com>

vbatts/oci-image-spec,opencontainers/image-spec,opencontainers/image-tools,vbatts/oci-image-spec,mattspencer-arm/image-spec,mattspencer-arm/image-spec,opencontainers/image-spec,xiekeyang/image-tools,jonboulle/image-spec,xiekeyang/image-tools,opencontainers/image-tools,jonboulle/image-spec