Add more information to enhancement: https://github.com/openshift/enhancements/blob/master/enhancements/api-review/add-new-CRD-ImageDigestMirrorSet-and-ImageTagMirrorSet-to-config.openshift.io.md

1. use conversion webhook to continue the support of the current ICSP
2. point out there will be two phases to land the new CRD:
   1. MCO implementation & conversion
   2. do not expose API until other components migrate to new CRDs.