spec: Restore compliance language
spec: Restore compliance language

This landed in 836fb1c5 ([ReadMe] Add Compliance Language, 2016-10-11,
#432) but was removed in 27508e2f (Clean-up spec to remove
non-spec-ish items, 2016-11-14, #458), likely accidentally during a
rebase.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

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