Document cargo-add's feature message

Document cargo-add's feature message

This refers to the `Features` part of the message that gets printed to
stderr after successfully adding some dependency, e.g.:

```
cargo add my-crate
    Updating crates.io index
      Adding my-crate v0.1.0 to dependencies.
             Features:
             + foo
             - bar
```

This tells us that the `foo` feature is enabled and `bar` is disabled.