Update SECURITY.md to mention GitHub Security Advisories

[rdimitrov]

The following PR updates the default SECURITY.md policy by adding the option of using GitHub Security Advisory to privately report security issues to all maintainers.

References:

• https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability

Signed-off-by: Radoslav Dimitrov dimitrovr@vmware.com

[rdimitrov]

cc: @tpepper @joshuagl @geekygirldawn

[tpepper]

lgtm