Document how to use with Rails API (thoughtbot#821)

docs/getting_started.md

@@ -42,7 +42,7 @@ visit <http://localhost:3000/admin> to see your new dashboard in action.
 
 ## Create Additional Dashboards
 
-In order to create additional dashboards, pass in the resource name to 
+In order to create additional dashboards, pass in the resource name to
 the dashboard generator. A dashboard and controller will be created.
 
 ```bash
@@ -58,3 +58,19 @@ namespace :admin do
   resources :foos
 end
 ```
+
+## Rails API
+
+Since Rails 5.0, we've been able to have API only applications. Yet, sometimes
+we still want to have an admin. To get this working, please update this config:
+
+```ruby
+# config/application.rb
+config.api_only = false
+```
+
+That means, when your app _boots_, we'll have access to flashes and such. We
+also don't use your `ApplicationController`. Instead, Administrate provides its
+own. Meaning you're free to specify `ActionController::API` as your parent
+controller to make sure no flash, session, or cookie middleware is used by your
+API.