rust: add README (#8242)

plugins/rust/README.md

+# rust
+
+This plugin adds completion for [`rustc`](https://doc.rust-lang.org/rustc/index.html), the compiler for the Rust programming language.
+
+To use it, add `rust` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... rust)
+```