adds some description re: compat with docker schemas
adds some description re: compat with docker schemas

Signed-off-by: Mike Brown <392d1074b9aea7041f97d21fe1e0b35c6b1de152@us.ibm.com>

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