docs: Add security policy

[karfau]

Inspired by the following sources:

• https://snyk.io/blog/ten-git-hub-security-best-practices/
• https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md
• https://expressjs.com/en/resources/contributing.html#security-policies-and-procedures
• https://docs.github.com/en/code-security/security-advisories/about-coordinated-disclosure-of-security-vulnerabilities

[karfau]

@brodybits @kethinov @timbru31 I would like to land this PR latest at the end of the month.

Let me know if you spot something that needs a change.

[timbru31]

Awesome, thank you. Will read through it this week 👍

[karfau]

@timbru31 Did you have a chance to read it? Do you need more time? Should I just land it?

[karfau]

Landing this now, since there was no update since nearly a month.