Add security bug reporting information to readme

[jeffpaul]

Description of the Change

This pull request adds important security reporting instructions to both the README.md and readme.txt documentation files for the Insert Special Characters plugin. The new section provides clear guidance on how to report security vulnerabilities, including a link to the Patchstack Vulnerability Disclosure Program.

Documentation updates for security reporting:

• Added a "Where do I report security bugs found in this plugin?" section to README.md, with instructions and a link to the Patchstack Vulnerability Disclosure Program for reporting security issues.
• Added a corresponding section to readme.txt, ensuring consistent security reporting guidance across documentation files.

How to test the Change

Changelog Entry

Developer - Add Patchstack security-reporting FAQ.

Credits

Props @jeffpaul.

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.