title: What's the use of route_options in Active Admin
body: <p>What's the exact use of active admin method route_options as</p>
  <pre><code>config.namespace :admin do |admin|
    admin.route_options = { path: '/admin', subdomain: 'admin' }
  end
  </code></pre>
  <p>Using rails 6, how we can use it to make a subdomain 'admin' to use all active_admin things? like</p>
  <p><a href="http://admin.localhost:3000/admin" rel="nofollow noreferrer">http://admin.localhost:3000/admin</a> ?</p>