Document relation to clap v3's derive feature

[dmarku]

Hi altogether!

I've noticed that clap 3.0.0 integrates StructOpt into its derive feature. Does this affect the future of StructOpt as a standalone project? If it's preferred to start new projects with clap + derive and migrate existing projects away from StructOpt, I'd like to suggest adding a short (deprecation) note to the top of the StructOpt Readme and docs.rs pages so that users who are directly visiting StructOpt documentation become aware of that and can act accordingly. I could set up an initial Pull Request if you wish. 🙂

For context: I started a small commandline app in Rust a few weeks ago following the Rust CLI book, which I'm pretty sure still referred to StructOpt at that time and has been updated to clap + derive since then (?). I encountered slightly odd behavior and looked through the issues of both StructOpt and clap to see if it is documented somehwere, where I found mentions of the merge into clap and finally discovered clap v3's release notes. They even have a short migration guide! 👍

Best regards and thanks for all your effort from a happy user!

[TeXitoi]

README updated, thanks.