Add README for the cargo plugin

plugins/cargo/README.md

+# cargo
+
+This plugin adds completion for the Rust build tool [`cargo`](https://github.com/rust-lang/cargo).
+
+To use it, add `cargo` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... cargo)
+```
+
+Updated on October 4th, 2016.