Document the Safetynet support

Summary

We need to explain the Safetynet implementation in /e/OS

What is the current behavior?

Users discovered Safetynet support with v1.0. We mention in the release note it works only for /stable, but it works also for some /dev devices.

What is the improved behavior?

Explain:

what is Safetynet
what we did
why we focused on /stable devices
- insist on the fact it hides the "root option", and disable some logs
what's the plan for other devices
(how to contribute)?

What does it bring?

Avoid confusion, and help community improve support on all devices