title: symfony2 entity relationship
body: <p>I try to make a social network with symfony2. I am not sure of how to modelize a user and his friends in the User entity.
  I mean a friend is also a user so I have a User entity linked to another User entity.</p>
  
  <p>How in the annotation of the User entity can I express that kind of relationship?
  For the moment I have something like :
  the User entity :</p>
  
  <pre><code>...
  /**
   * @ORM\ManyToMany(targetEntity="User")
   *
   */
  private $friends;
  ...
  </code></pre>
  
  <p>with $friends as an arraycollection. Is it correct?</p>