doc: move TFSEC section from the main README.md into separate file

[mglotov]

Prerequisites

Please answer the following questions for yourself before submitting an issue. YOU MAY DELETE THE PREREQUISITES SECTION.

• I am running the latest version
• I checked the documentation and found no answer
• I checked to make sure that this issue has not already been filed

Expected Behavior

It'd be better to have a separate file with TFSEC notes

Current Behavior

TFSEC section is large now and it's hard to keep README.md file clean and readable.