Add Security Policy

Adding a Security Policy is important to provide guidance on how users can report potential vulnerabilities and communicate when vulnerabilities will be confirmed, fixed and disclosed to the public. When possible, vulnerabilities should be reported privately to avoid being exploited by attackers. It could have been useful in this case https://github.com/boostorg/boost/issues/747 for example.

This is considered a good-practice and recommended by [Github](https://docs.github.com/en/code-security/getting-started/securing-your-repository#setting-a-security-policy) and [Scorecard](https://github.com/ossf/scorecard).

If you agree with this addition, I can support you by opening a PR with a Security Policy draft.

##### Additional Context
Hi! I'm Gabriela and I work on behalf of Google and the OpenSSF suggesting supply-chain security changes :)