title: What does _sonata_admin: {resource: . } do in routing.yml?
body: <p>When setting up SonataAdmin, you have to add an entry like this one:</p>
  
  <pre><code>_sonata_admin:
      resource: .
      type: sonata_admin
      prefix: /admin
  </code></pre>
  
  <p>In this entry, what does <code>resource: .</code> do? It's somewhat unusual syntax and I'd like to add a footnote about it to the SonataAdmin docs.</p>