Add upstream oh_my_zsh repo

zsh/plugins/rust/README.md

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+# 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)
+```