Document PSBT roles and limitation

Author: DanGould

README.md

@@ -26,7 +26,7 @@ Supports (or should support)
 * De/serialization of blocks and transactions
 * Script de/serialization
 * Private keys and address creation, de/serialization and validation (including full BIP32 support)
-* PSBT creation, manipulation, merging and finalization
+* PSBT v0 de/serialization and all but the Input Finalizer role. Use [rust-miniscript](https://docs.rs/miniscript/latest/miniscript/psbt/index.html) to finalize.
 * Pay-to-contract support as in Appendix A of the [Blockstream sidechains whitepaper](https://www.blockstream.com/sidechains.pdf)
 
 For JSONRPC interaction with Bitcoin Core, it is recommended to use