[DOC] Create documentation for Security.txt

[sdzhepa]

Required types of documentation:

• README.md
• Magento DevDoc
• Magento User Docs (if applicable)

Note:

• an approach, where should be placed documentation, will be defined later(as a part of the module or as a page on Dev/Merch docs)

Links and materials

• https://tools.ietf.org/html/draft-foudil-securitytxt-09
• https://github.com/magento/security-package/blob/1.0-develop/Securitytxt/README.md