[DDC-3087] Add readonly attribute to Field ORM Mapping to avoid generating a setter

The change allows one to enter a field attribute called "readonly". What this does is when using doctrine:generate:entities, only a getter is generated, setter is omitted. 

Example use on a created field that uses Timestampable and should not have a setter:
    /**
     * @var /DateTime $createdAt
     *
     * @Gedmo\Timestampable(on="create")
     * @ORM\Column(type="datetime", readonly=true)
     */
    private $createdAt;