Adding security advisories

[mags0ft]

As rubedo is an Anti-Cheat system for Minecraft, it would be highly effective to open any critical security advisories using GitHubs built-in feature for private vulnerability disclosure.

Hence, players won't abuse a crucial security bug until it's fixed and can be published. As well, this repo is missing a SECURITY.md file which would help users understand how to report security issues.

Advisories can be activated under Security > Vulnerability reporting > Enable.

[Apoorslime]

Good issue