chore: add security policy

[gruyaume]

Description

Here we add the security policy as it is now mandated for every public repository.

Rationale

Any public repository must also include a SECURITY.md file in the root directory, which educates users/contributors on how to report a security concern. An example template is here and the LXD SECURITY.md file can be seen here as a reference.

Reference

• https://docs.google.com/document/d/1RolsiWCWhM-sVmHFjLt-Mq5yKcgQJ8GLXssGJREjSL0/edit
• https://docs.google.com/document/d/1wAO9ejAYcTpG76xwR822RZCnxfTMkf_caODWUL7BldE/edit?pli=1

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.