Add notice about libbitcoinconsensus deprecation

[cylewitruk]

Hi!

It would be good if you added a notice to this repo that Bitcoin Core's libbitcoinconsensus was deprecated in 27.0 and removed in 28.0, so that users are aware of this (particularly that taproot was its last consensus update) before pulling this crate into new projects.

It sounds like libbitcoinkernel is the intended path forward beyond taproot?

[apoelstra]

cc #95

cc rust-bitcoin/rust-bitcoin#2973

Yep, for sure, we should update the README here. Currently it has no mention of any of this.