Add a Security-Policy

[diogoteles08]

Hi @klauspost! I'm Diogo and I'm back (see #764) hoping to offer a bit more help with security enhancements.

This time I'm here to suggest that you expose a way that users can report eventual vulnerabilities in a safe and efficient way. This is usually done through a Security Policy, which is a GitHub standard document (SECURITY.md) added on the root of the repo and will be visible to the users in the "Security Tab".

It is a recommendation from Github itself, and from Scorecard (being a security measure of medium priority).

Aiming to make this change easier, I'll take the liberty of submitting one suggestion of a Security Policy as a PR. Please feel free to edit it directly or ask me for editions until it is in compliance with how klaupost/compress would best handle vulnerability reports.

[klauspost]

Good idea. Added a bit more, specific to the packages here.