README.md in ember-auth-rails-3.8.0 vs README.md in ember-auth-rails-4.0.0

@@ -19,9 +19,13 @@
 
 Add `ember-auth` to sprockets. `application.js`:
 
     //= require ember-auth
 
+Add the `ember-auth-source` gem to control dependencies explicitly:
+
+    $ gem install ember-auth-source --version '3.0.0'
+
 ## Usage
 
 See [README](https://github.com/heartsentwined/ember-auth) at the main
 `ember-auth` site.