added transaction option to bulkCreate docs

lib/model.js

@@ -1252,6 +1252,7 @@ module.exports = (function() {
    * @param  {Boolean}      [options.individualHooks=false]  Run before / after create hooks for each individual Instance? BulkCreate hooks will still be run if options.hooks is true.
    * @param  {Boolean}      [options.ignoreDuplicates=false] Ignore duplicate values for primary keys? (not supported by postgres)
    * @param  {Array}        [options.updateOnDuplicate]      Fields to update if row key already exists (on duplicate key update)? (only supported by mysql & mariadb). By default, all fields are updated.
+   * @param  {Transaction}  [options.transaction]
    *
    * @return {Promise<Array<Instance>>}
    */