docs: add installation instructions

[clouedoc]

I am a regular Ledger user looking to safely generate Shamir Secret keys.
It took me a while to understand how to load it on my device.
I hope this helps the next hobbyist :)

[aido]

Hi @clouedoc,

Thanks for the PR.

The app is currently going through a security audit and hopefully soon will be available to install from Ledger Live. This is how most people will install the app. Sideloading will be for more advanced uses who want the latest version released on GitHub.

Also, I'd rather most people do not trust some random person on GitHub (me) by sideloading the app and instead install the audited app through Ledger Live.

Instead of having your instructions in the main README I may put them in a separate instruction for more advanced users explaining the risks involved with sideloading apps that may not have been audited.

[aido]

See here for a discussion on app-seed-tools progress through the Ledger audit process:

#15