Very slight `cargo add` documentation improvements
As discussed in https://github.com/rust-lang/book/pull/3331, a quick explanation of the `Features` part of the message that gets printed to stdout when adding some dependency.
Consider the following example:
```
cargo add my-crate
Updating crates.io index
Adding my-crate v0.1.0 to dependencies.
Features:
+ foo
- bar
```
It was not clear to me what `+foo` and `-bar` meant until `@carols10cents'` kindly explained it to me. Hopefully the documentation now clarifies this.
TODO:
- [x] Run `./build-man.sh`