Added class and os alternates

.oh-my-zsh/plugins/heroku/README.md

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+# Heroku
+
+This plugin provides completion for the [Heroku CLI](https://devcenter.heroku.com/articles/heroku-cli).
+
+To use it add heroku to the plugins array in your zshrc file:
+
+```bash
+plugins=(... heroku)
+```