Add an example PSBT workflow. The workflow we simulate is that of a
setup using a watch-only online wallet (contains only public keys) and a
cold-storage wallet (contains the private keys).
We create and update a PSBT using the watch-only wallet then pass the
PSBT to the cold-storage wallet to sign.
Co-authored-by: Dan Gould <d@ngould.dev>
Tobin Harding