README: remove Maintainers
README: remove Maintainers

Redundant with MAINTAINERS file.

Signed-off-by: Jonathan Boulle <0d47477beef1d882e8812885f3dcf66c4a47c346@gmail.com>

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