Doc change - custom auto-generated values

Updated documentation to include mention of returning values from a function for primary keys.

lib/data-types.js

@@ -36,6 +36,20 @@ var util = require('util')
  *   }
  *   }
  * })
  * })
  * ```
  * ```
+ * There may be times when you want to generate your own UUID conforming to some other algorithm. This is accomplised
+ * using the defaultValue property as well, but instead of specifying one of the supplied UUID types, you return a value 
+ * from a function.
+ * ```js
+ * sequelize.define('model', {
+ *   uuid: {
+ *     type: DataTypes.UUID,
+ *     defaultValue: function() {
+ *       return generateMyId()
+ *     },
+ *     primaryKey: true
+ *   }
+ * })
+ * ```
  *
  *
  * @class DataTypes
  * @class DataTypes
  */
  */