title: How to display total number of users on Active Admin User page (rails 3.2, active admin gem)
body: <p>I managed to install active admin and I can see the users with the following table:</p>
  
  <pre><code>ActiveAdmin.register User do
  
  index do   
      selectable_column                         
  column :email                     
  column :created_at
  column :confirmed_at  
  column :last_sign_in_at           
  column :last_sign_in_ip
  column :sign_in_count     
  
  default_actions                   
  end  
  
  filter :email
  filter :created_at
  filter :confirmed_at
  filter :last_sign_in_at 
  filter :sign_in_count
  
  end
  </code></pre>
  
  <p>I would like at the top of my page to display the total number of users. </p>
  
  <p>How can I do it? Should I do it on admin/user.rb or modify another page (a view page?)</p>