title: Custom render in generated admin with Symfony
body: <p>So, here's my little problem: I have generated an admin with the Symfony admin generator, but I need to display a link (or a button for that matter) that can be click to open a C.V. file.</p>
  
  <p>As of now, i have:</p>
  
  <pre><code>generator:
    class: sfDoctrineGenerator
    param:
      model_class:           Client
      theme:                 admin
      non_verbose_templates: true
      with_show:             false
      singular:              ~
      plural:                ~
      route_prefix:          client
      with_doctrine_route:   true
      actions_base_class:    sfActions
  
      config:
        actions: ~
        fields:
           job_titles_list: { label: Emplois disponnibles }
           created_at:      { label: Cree le }
           updated_at:      { label: Mis a jour le }
        list:    
           display: [=name, firstname, experience, connexe, formation, created_at, cv_file]
           title: Gestion des prospects      
        filter: 
           display: [name, firstname, phone, cellphone, postalcode, experience, connexe, formation, job_titles_list, created_at, updated_at]
        form:    ~
        edit:  
           fields:
              formation: { label: Formation }
              experience: { label: Experience }
              connexe: { label: Connexe }
        new:     ~
  </code></pre>
  
  <p>As you can see, in list/display, i have a cv_file, which gives me a link (from the DB). I need to render this column with a link.</p>
  
  <p>By the way, I know nothing about Symfony, so be gentle ;)</p>
  
  <p>Thanks a lot!</p>