lein: add README (#8189)

plugins/lein/README.md

+# Leiningen plugin
+
+This plugin adds completions for the [Leiningen](https://leiningen.org/) Clojure build tool.
+
+To use it, add `lein` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... lein)
+```