codeclimate: add README

plugins/codeclimate/README.md

+# codeclimate plugin
+
+This plugin adds autocompletion for the [`codeclimate` CLI](https://github.com/codeclimate/codeclimate).
+
+To use it, add `codeclimate` to the plugins array in your zshrc file:
+```zsh
+plugins=(... codeclimate)
+```