docs(readme): Link out to shadow for long_version

This seems like a case where we shouldn't solve all the problems but
point to others who do.

Fixes #1363

README.md

@@ -119,6 +119,7 @@ Why use the procedural [Builder API](https://github.com/clap-rs/clap/blob/v3.0.1
 
 - [wild](https://crates.io/crates/wild) for supporting wildcards (`*`) on Windows like you do Linux
 - [argfile](https://crates.io/crates/argfile) for loading additional arguments from a file (aka response files)
+- [shadow-rs](https://crates.io/crates/shadow-rs) for generating `App::long_version`
 - [clap_complete](https://crates.io/crates/clap_complete) for shell completion support
 - [clap-verbosity-flag](https://crates.io/crates/clap-verbosity-flag)
 - [clap-cargo](https://crates.io/crates/clap-cargo)