PSA: Important info about rustc's new feature staging

@bluss Ah, interesting. I'm inclined to agree with you that deny(unused_features) by default is too punitive here.

Note that to compile with the beta or stable compilers the feature attribute can't be written at all, so it is expected that crates have to take a transitional step of removing all feature attributes before being compatible with the stable compiler.

Here's an issue unused_features warn by default? · Issue #21798 · rust-lang/rust · GitHub