Update Ledger simulator instructions for local testing

[Sjors]

There's partial instructions here for the ledger emulator: https://github.com/bitcoin-core/HWI/tree/master/test#ledger-emulator

But it doesn't explain how to build the app and give the emulator access to it.

I was able to build the app with Docker (on Ubuntu): https://github.com/LedgerHQ/ledger-app-builder

And even to run it the simulator from that container.

But then how do I access that with ./run_tests.py --device-only --ledger? Is it a matter of providing the right --ledger-path? Or do I have to run HWI itself inside the Ledger docker container?

cc @bigspider