heroku: add README (#7342)

plugins/heroku/README.md

@@ -0,0 +1,9 @@
+# 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)
+```