Package signing issues with pacman 4.x ?

When updating my system today, I got this error:

error: i3lock: signature from "Thorsten Töpper "
is unknown trust
error: failed to commit transaction (invalid or corrupted package)
Errors occurred, no packages were upgraded.

Whoa! I started googling and of course I found the answer, as
Arch is one of the best documented distros, next to Gentoo.
In short, after the release of pacman 4.x (check with pacman -V)
, one has to change the options to one’s enabled repos (in /etc/pacman.conf)
from SigLevel = PackageRequired to SigLevel = Optional TrustAll.
For more info, check here.