README: remove the roadmap
README: remove the roadmap

We have the roadmap encoded into GitHub issues. Just point there
instead.

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

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