[SAC] Userid mantra changed

As at least 2 new users [1] were found spamming,
the mantra was updated.

You can read the new mantra here (requires sign-in):
https://git.osgeo.org/gogs/sac/userid-mantra/wiki/Home
Or by logging via ssh on the www host.

[1] https://trac.osgeo.org/osgeo/ticket/1739#comment:26

--strk;