Add comments explaining each dependency

[nnmm]

I got the idea from this reddit post: https://www.reddit.com/r/rust/comments/u5cjmu/dependency_justification_section_may_need_more/

[jhdcs]

Looks good, but should there be comments for the examples crate as well? To fully document everything going on for ROS 2 users not used to Rust (and vice-versa)? Or would this be repeating ourselves too much?

[nnmm]

Looks good, but should there be comments for the examples crate as well? To fully document everything going on for ROS 2 users not used to Rust (and vice-versa)? Or would this be repeating ourselves too much?

I looked at the rclrs_examples/Cargo.toml but decided not to comment it for now. It's just anyhow + message packages, so there's nothing interesting to say about these, and I also think this convention applies more to libraries than binaries. If you'd like me to, I can add it though.

[jhdcs]

If you feel that adding the extra comments will be too confusing/verbose, then I will defer to your judgement. Otherwise, everything looked good to me!

[nnmm]

Thanks!