document how it works

[michielbdejong]

• alice.sendMoney is called
• Alice does a BTP Message request with sub-protocol '_get_lightning_invoice' and protocolData, the amount in Satoshi, as a String.
• Bob responds with a JSON { paymentRequest: ... }
• Alice pays the paymentRequest and
• Alice sends a BTP Transfer request, this time protocolName is 'payment_preimage', and protocolData is JSON { paymentPreimage }

[michielbdejong]

the current readme is still correct for the ilp-plugin-lightning that's on npm, but we should update it for the new ilp-plugin-lnd-asym-client role.

[michielbdejong]

(note that the BTP Transfer decreases Alice's unsecured amount instead of increasing it, due to the inverted transfer logic in BTP/2.0 compared to BTP/1.0)