Add placeholder for documenting exports from an addon

Towards advancing [RFC ember-cli#779](emberjs/rfcs#1059)

Author: kategengler

packages/addon-blueprint/files/README.md

@@ -18,6 +18,16 @@ ember install <%= addonName %>
 
 [Longer description of how to use the addon in apps.]
 
+> TODO: Document the package's public API.
+>
+> For each public api (including components, helpers, modifiers, and other apis) include:
+>
+> - The import path for a consumer (e.g. `import MyAddonsComponent from 'my-addon/components/my-addons-component'`)
+> - What it does
+> - Parameters/options
+> - Return value
+> - Example usage
+
 ## Contributing
 
 See the [Contributing](CONTRIBUTING.md) guide for details.