Add readme.txt FAQ entry for filing security bugs

[felixarntz]

What?

This adds an FAQ entry for where to file security bugs.

Why?

Per request from the WordPress security team.

How?

The copy used for the content is the same that is present here in the SECURITY.md file.

[peterwilsoncc]

Thanks Felix, much appreciated.

[github-actions]

Flaky tests detected in a49ad52.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4442300614
📝 Reported issues:

• [Flaky Test] doesn't throw a block error when clicking on a link #47747 in /test/e2e/specs/site-editor/template-part.spec.js