isbn-verifier: Include more succinct description of ISBN-10
isbn-verifier: Include more succinct description of ISBN-10

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

exercism/xhaskell