* Clap has dependencies even with all features disabled.
* The "nightly" feature does nothing, so don't mention it.
* Explain the difference between "unstable" and "nightly" better.
* Split features into groups (default, opt-in, clap-development).
* For contributors: update what tests should be run, and remove make command.
* Removes a repeated word and splits up some long markdown lines.
Also groups features by category and dependencies by feature in Cargo.toml.
