Proposal: I think we should remove theextern crate declaration in the README, or at least put a comment above it noting that it is only necessary if you're on the Rust 2015 edition.

Background: I am a mentor onexercism.io, and our first "real" exercise (after "Hello, World") is the "Reverse String" exercise. We have a "bonus" test case where a string contains "characters" that are multiple code points. Most students rightly find this crate and use it to extract the graphemes from a string and reverse those. However, many of them leave in theextern crate declaration, even though essentially all of them are using Rust 2018 and don't need it.