Enhancement: Add `SECURITY.md`

[localheinz]

What is the reason for this PR?

• [x] add a SECURITY.md

Also see #740 and #744.

💁‍♂️ While the primary purpose of a SECURITY.md is to inform users of fakerphp/faker about our security policy, this file could also serve well as a starting point for a discussion which versions of fakerphp/faker we currently support.

I may have missed it, but I have not found such a document in this repository.

Author's checklist

• [x] Follow the Contribution Guide

Summary of changes

Review checklist

• [x] All checks have passed
• [ ] Changes are added to the CHANGELOG.md
• [ ] Changes are approved by maintainer

[localheinz]

Thank you, @bram-pkg, @GrahamCampbell, and @pimjansen!