Document P2P Transactions

[ty-everett]

I was originally going to make a pull request that added P2P transaction broadcasting and payment destination support to this library.

But then, after reviewing the source code, I realized that it was already added! It just hasn't been documented yet.

I think that having documentation around P2P Transactions in this library (and P2P Transactions more generally) is important for adoption and scalability. We should really emphasize it to developers, especially new developers building solutions. When scanning no longer becomes practical, having P2P infrastructure in place will ensure that as many services as possible can switch as gracefully as possible.

I'm happy to take the time to document this, update README.md and add inline code docs (and there is at least one typo in the example code I'd like to fix), but it looks like there are already 5 PRs on this library which haven't been merged. I am hesitant to propose new changes if they won't be merged, and I'm not sure if this library is actively maintained.