Adding guidenance to prevent users giving themselves admin role

[Ornias1993]

Currently the guide stops at setting up a Proof of Concept, where users can decide being an admin at signup.

It would be nice to add a small expantion which covers both security rule(s) to prevent users from setting themselves to anything other than Role=User, and creating a small addon to the admin backend that allows admins to setup others as admin.

I know it is more than two "small" features, I managed to do it but it took some time with the limited amount of good information on the internet. So I guess more people would be interested in guidence from a trusted source :)