Update mirror.md
Update mirror.md

It was not obvious that additional configuration is needed for the official Registry image.

cc @dmcgowan @stevvooe
JimGalasyn/docker.github.io,anweiss/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,johnstep/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,johnstep/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,JimGalasyn/docker.github.io,JimGalasyn/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,anweiss/docker.github.io,johnstep/docker.github.io,anweiss/docker.github.io,JimGalasyn/docker.github.io,johnstep/docker.github.io,anweiss/docker.github.io,docker/docker.github.io,JimGalasyn/docker.github.io,anweiss/docker.github.io,johnstep/docker.github.io