Add note about ESDoc generator for classic classes

Author: samselikoff

tests/dummy/app/pods/docs/quickstart/template.md

@@ -13,6 +13,8 @@ marketing/demo page you can use to promote your addon.
 
 2. **Install an API doc plugin.** API docs are autogenerated from comments in your source code. Depending on which style you'll be using ([ESDoc](https://esdoc.org/) or [YUIDoc](http://yui.github.io/yuidoc/)), install the corresponding addon-docs plugin.
 
+    Note that the ESDoc generator is for native classes only. If you're using classic classes (Ember.Object.extend()), you can use the YUIDoc adapter.
+
   ```
   # ESDoc
   ember install ember-cli-addon-docs-esdoc