Document PSBT roles and limitation

[DanGould]

The READEME claims rust-bitcoin supports PSBT finalization, but really needs rust-miniscript for that. I think we should make this clear in this crate's PSBT examples as well.

Understanding scripts and witnesses doesn't scream rust-miniscript to me. Miniscript crate is not the first place I'd look, since it's additional.

We should probably add it to the README here or something. If I needed to deal with scripts or witnesses in any way beyond reserializing them or hashing scriptpubkeys into addresses, I'd immediately turn to Miniscript. You basically can't work on scripts without it, unless you are doing something very specific.

Anywhere there's PSBT code I expect a finalizer, even if it had documented limitations.

Not to be too glib, but suppose we had a finalizer that just always returned an error, and was documented not to finalize any transactions. Would that meet your expectations? It is hard to do much more without Miniscript.

Originally posted by apoelstra in #630 (comment)

[apoelstra]

Do we really only support PSBT v0? Otherwise this looks good to me.

[DanGould]

We only support v0 for now. Any non zero version number errors as per bip-174 version numbers spec

rust-bitcoin/src/util/psbt/map/global.rs

Lines 163 to 182 in 9cfa9bd

	if pair.key.key.is_empty() {
	// there can only be one version
	if version.is_none() {
	let vlen: usize = pair.value.len();
	let mut decoder = Cursor::new(pair.value);
	if vlen != 4 {
	return Err(encode::Error::ParseFailed("Wrong global version value length (must be 4 bytes)"))
	}
	version = Some(Decodable::consensus_decode(&mut decoder)?);
	// We only understand version 0 PSBTs. According to BIP-174 we
	// should throw an error if we see anything other than version 0.
	if version != Some(0) {
	return Err(encode::Error::ParseFailed("PSBT versions greater than 0 are not supported"))
	}
	} else {
	return Err(Error::DuplicateKey(pair.key).into())
	}
	} else {
	return Err(Error::InvalidKey(pair.key).into())
	}

[apoelstra]

ACK 896ca42

[tcharding]

ACK 896ca42