Document extensions in readme

[robinkrahl]

The extension mechanism and our bundled extension , otp-cache, is currently a bit hard to discover. We should mention it in the readme and refer to the man page for implementation details.

Also, we could list other extensions not bundled in the repository. Currently, I’m only aware of my own:

• nitrocli-nk3 (provisional until we have proper nk3 support in nitrokey-rs and nitrocli)
• nitrocli-otp-qr (provisional as we talked about including it in this repository)

[d-e-s-o]

Sounds good, feel free to submit a pull request.