Write useful documentation

[Follpvosten]

The documentation of this crate is currently rather poor; the only doc comments are in the mdc_sys module, and they are copied directly from the MDC docs.

Ideally, there would be crate-level docs that produce the README (#1) using cargo-readme, as well as module-level docs per component with more detailed usage examples.

Publishing docs will be a separate issue (which will get relevant once I'm at a version 0.1.0)