title: activeadmin: nested resources belongs_to
body: <p>I need to make nested resources in activeadmin like</p>
  
  <pre><code>http://localhost:3000/admin/companies/1/admin_documents
  </code></pre>
  
  <p>I tried</p>
  
  <pre><code>ActiveAdmin.register AdminDocument do
  
    belongs_to :company
  
  end
  </code></pre>
  
  <p>But there is no result, I get an error <code>No route matches [GET] "/admin/companies/1/admin_documents"</code></p>
  
  <p>What am I doing wrong? Thanks!</p>