Document Unstable Rust

[nyarly]

Attempting to install from source, I run into rust-lang/rust#87335 - fair enough, but it'd be nice to know that up front in the README or with a badge

[clux]

Hello there. This does actually build with stable rust. See e.g. the ci job that uses stable ci image.

We use nightly for rustdoc just like the main repo, but you can totally build with stable. Be sure to rustup update stable!