Improve crate-level documentation and `README.md`

[Techcable]

Improve the crate-level documentation and README.md

Right now the crate-level documentation is just a single sentence.
The README.md could use improvement too.

Maybe we can use cargo-sync-rdme to sync them.

[Techcable]

Synced crate docs & readme with 756471a (using cargo-rdme)

The docs could still use improvement though.