docs: Improve Crate Documentation

[carlaKC]

simln-lib is a crate now 🎉
But it's quite an unfriendly one (not- 🎉 ) becuase we're missing a few docs.

There are a few low-hanging fruit that we could address to fix this up.

Add a Readme

Better documentation for exported structs / enums

[sangbida]

Hey @carlaKC
I'm happy to pick up this one next, if you haven't started work on it yet. I'm happy to pick up something else or do some code reviews in the meantime if you've made progress on this already :)

[carlaKC]

Sure, feel free to get going on this!

[sangbida]

Hello, just thought I'd drop an update here. I've been working on this slowly here: #241

The plan is to get some cln nodes up and running and create a test project to validate the Usage setup that I'm putting in and continue adding docs for the structs, traits and enums.

[carlaKC]

Made a really good start on this!

Leaving this open with the stretch goal of turning on #![deny(missing_docs)] for both simln-lib and sim-cli (+ adding all the required comments that we need to get our checks to pass with this enabled).