isbn-verifier: Include more succinct description of ISBN-10 (#417)
isbn-verifier: Include more succinct description of ISBN-10 (#417)

Fixes https://github.com/exercism/problem-specifications/issues/1011.

A simple definition of what the system should do before getting in to
exactly what the digits of an ISBN mean would be good, in my opinion.

https://github.com/exercism/problem-specifications/pull/1019
petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,exercism/xrust