Start on basic `libipld` crate-level docs.

[nathan-at-least]

I just came to this repo via https://ipld.io/libraries/rust/ and found the lack of crate docs an inhibitor. I started basic crate docs with info about serde and codec deprecation (#164) to help newcomers get started.

If I find more time I'd like to document the rest of the API more thoroughly as I learn its design.