Better note limitations and purpose of the hash

[Noratrieb]

It came up in #56 (comment) that someone had problems because the hash broke over a minor release. I think that's fine for the intended purpose of the hash (hashmaps), but this should be better explained in the readme.

[sstadick]

@Noratrieb - The Rust Performance book specifically points to using this: https://nnethercote.github.io/perf-book/hashing.html

Additionally, there are nearly 1000 other crates that depend on this: https://lib.rs/crates/rustc-hash

This change also broke my project. Given this crate is under the rust-lang org, I don't think it's too onerous to ask that it follows semver.