add in readme.md few lines about defautl admin pass

[arhimede]

in order to avoid AI slops and waisting the time of the mainainters

like the below one
https://github.com/dotkernel/api/security/advisories/GHSA-vhxj-xfgm-xf8r

let's add a short paraghrah in readme.md about default passwords and a red warning to chjange it in production