Allow iframe to docs.rs

[SoniEx2]

iframe to docs.rs is a better experience than including README.md as crate docs.

supports code links too. e.g.

//! This is a crate comment with a link to [`String`]. This doesn't work in a README.md

[Turbo87]

you can achieve the same thing with a regular markdown link to the Rust docs. This would be a pretty drastic change, which seems unlikely to happen at this point.