Add documentation on how to report a security vulnerability

[jayfk]

based on https://github.com/pyupio/safety-db/issues/2262

[Jwomers]

@jayfk great idea! Will work in this in Q1!

[remram44]

For Ruby, there is https://github.com/rubysec/ruby-advisory-db

For Rust, there is https://github.com/RustSec/advisory-db

This does not seem to have any way to add vulnerabilities.