docs: add note about needing ember-auto-import

Related to discussion in #14

README.md

@@ -13,11 +13,13 @@ Stencil provides a great, opinionated process for defining Web Components (custo
 Start by installing your Stencil components as `npm` modules, as described in the [distribution instructions][distribution] in the guide. Then, install this addon:
 
 ```bash
-ember install ember-cli-stencil
+ember install ember-cli-stencil ember-auto-import
 ```
 
 That's it! Your Stencil components will automatically be detected by the addon and pulled into your application.
 
+**Note:** `ember-auto-import` is _required_ for this addon to work correctly.
+
 ## Usage
 
 Since Stencil components are detected automatically, you can just start using any Stencil components discovered in your dependencies without any further configuration required. Props can be passed to them, just like other elements, and events listened to through the [`{{on}}` modifier][on-modifier].