Document `lightning` crate features

[TheBlueMatt]

We have a few features defined in Cargo.toml with comments describing their use, but we don't expose them in the crate-level lib.rs documentation, making docs pretty much invisible. We should copy the text over, expand it, and clarify it so that you can see the features on docs.rs.

[mattfaltyn]

I can look into this one if no one else is working on it :)