Update README.md for engines

As mentioned in #134

Author: k-fish

Diff for: README.md

@@ -96,6 +96,24 @@ The major breaking changes to update to 3.x are so far:
 - Test helpers are now all covered by `setBreakpoint`
 - Calling media breakpoints in templates is now done with a helper. `{{media.isDesktop}}` -> `{{media 'isDesktop'}}`
 
+## Usage in engines
+
+If you are using engines and you want to share responsive behaviour between the main application and engine, you must pass the 'media' service to the engine app.
+
+```js
+const Eng = Engine.extend({
+  modulePrefix,
+  podModulePrefix,
+  Resolver,
+  dependencies: {
+    services: [
+      'media',
+      ...
+    ],
+  },
+});
+```
+
 ## Testing Helpers
 This project provides a single test helper which works in both integration and acceptance tests to assist in testing
 content specific to different breakpoints.