unsynced Clocks produces nbf to fail as it issued on before time actual on registry

[kalabiyau]

[mssola]

As discussed on IRC, this is not really an issue from our side. Just use NTP to keep all the clocks on sync.

[kalabiyau]

@mssola at least worth a mention on the README, I guess

[mssola]

@kalabiyau done! https://github.com/SUSE/Portus/wiki/Synchronizing-the-Registry-and-Portus#synchronizing-clocks-between-the-registry-and-portus