title: adding created timestamp to join table in doctrine2
body: <p>I have the following property in my User entity to track followers and following. Basically a user can follow other user as well. I have a join column called app_user_follow_user, however I also wanted to add a timestamp of whenever someone follows another user, when did it happen. How can I specify a created timestamp via this ORM?</p>
  
  <pre><code>/**
   * @ORM\ManyToMany(targetEntity="User", mappedBy="following")
   */
  protected $followers;
  
  /**
   * @ORM\ManyToMany(targetEntity="User", inversedBy="followers")
   * @ORM\JoinTable(name="app_user_follow_user",
   *      joinColumns={@ORM\JoinColumn(name="user_id", referencedColumnName="id")},
   *      inverseJoinColumns={@ORM\JoinColumn(name="follow_user_id", referencedColumnName="id")}
   *      )
   */
  protected $following;
  </code></pre>