Clarify BTP version

[michielbdejong]

In the readme, we should explain that this plugin implements BTP/2.0 (if that's what we want to call it), and is incompatible with BTP/1.0.

[michielbdejong]

Also, we need to explicitly document (since it's so confusing) that the new Transfer/Response/Error sequence decreases (settles) the sender's unsecuredAmount. This is the opposite of how in BTP/1.0 the Prepare/Fulfill/Reject sequence used to increase (cause) the sender's unsecuredAmount,
see interledger/rfcs#360 (comment). It would be easy to oversee this difference, and think that Transfer/Response/Error is just the unconditional version of Prepare/Fulfill/Reject. Or maybe we should just say "forget everything you think you know about BTP". :)