title: activeadmin: adding delete for a nested resource
body: <p>I have a infrastructure object composed for many datacenters. In the apps/admin/infrastructures.rb I have the following code:</p>
  
  <pre><code>form do |f|
    f.inputs "Infrastructure details" do
      f.input :name
  
      f.has_many :datacenters do |datacenter_form|
        datacenter_form.input :name        
      end
    end
    f.buttons
  end
  </code></pre>
  
  <p>I can add datacenters with no problems but I don't know how I can delete it from infrastructure form.</p>