Fix #177 Update README.md for Security.txt module

[sdzhepa]

Description (*)

Fixed Issues (if relevant)

1. Update README.md for Security.txt module #177: Update README.md for Security.txt module

Manual testing scenarios (*)

Compare old version vs new

1. OLD: https://github.com/magento/security-package/blob/1.0-develop/Securitytxt/README.md
2. NEW: https://github.com/magento/security-package/blob/fix_issue_177/Securitytxt/README.md

Questions or comments

Contribution checklist (*)

• Author has signed the Adobe CLA
• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)