README: clean-up and clarifications
README: clean-up and clarifications

- Remove redundant sentence describing media types document
- Disambiguate "OCI Layers", and more directly explain link to archival
  format
- Change ROADMAP pointer to a more realistic date
- Minor tweaks to FAQ wording

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

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