[ci skip] Add BTM timestamps option to api doc

lib/associations/mixin.js

@@ -275,6 +275,7 @@ Mixin.hasMany = function(target, options) { // testhint options:none
  * @param {string|object}       [options.foreignKey] The name of the foreign key in the join table (representing the source model) or an object representing the type definition for the foreign column (see `Sequelize.define` for syntax). When using an object, you can add a `name` property to set the name of the colum. Defaults to the name of source + primary key of source
  * @param {string|object}       [options.otherKey] The name of the foreign key in the join table (representing the target model) or an object representing the type definition for the other column (see `Sequelize.define` for syntax). When using an object, you can add a `name` property to set the name of the colum. Defaults to the name of target + primary key of target
  * @param {object}              [options.scope] A key/value set that will be used for association create and find defaults on the target. (sqlite not supported for N:M)
+ * @param {boolean}             [options.timestamps=sequelize.options.timestamps] Should the join model have timestamps
  * @param {string}              [options.onDelete='SET NULL|CASCADE'] Cascade if this is a n:m, and set null if it is a 1:m
  * @param {string}              [options.onUpdate='CASCADE']
  * @param {boolean}             [options.constraints=true] Should on update and on delete constraints be enabled on the foreign key.