Update docs front page

[spacebear21]

Currently the first thing one sees on https://docs.rs/payjoin/latest/payjoin/ is:

Important: this crate is WIP!

While I don’t think there’s a huge risk running it, don’t rely on its security for now! Please at least review the code that verifies there’s no overpayment and let me know you did.

This is a library and an example binary implementing BIP78 Payjoin. The library is perfectly IO-agnostic - in fact, it does no IO. The primary goal of such design is to make it easy to unit test. While we’re not there yet, it already has infinitely more tests than the Payjoin PR against Electrum. :P

Additional advantage is it doesn’t care whether you use async, blocking, tokio, sync-std hyper, actix or whatever. There are already too many frameworks in Rust so it’s best avoiding directly introducing them into library code.

To use this library as a sender (client, payer), you need to enable send Cargo feature.

To use this library as a receiver (server, payee), you need to enable receive Cargo feature.

I'm not yet advocating getting rid of the "still a WIP" disclaimer, but a lot of info there is out of date.

• BIP78 is supported but no longer the primary function of the library.
• Comparison with a long-forgotten PR against Electrum is unnecessary.
• sender and receiver features no longer exist.

[arminsabouri]

BIP78 is supported but no longer the primary function of the library.

We should also update payjoin/Cargo.toml

[DanGould]

While making this edit

//! The library is perfectly IO-agnostic - in fact, it does no IO.
//! The primary goal of such design is to make it easy to unit test.

I think there are other reasons for this design (separation of concerns, abstraction, encapsulation), but I am curious if we're able to actually unit test what we have with this design.

I don't think we need to note the goal in the readme.