Describe how to toggle myth options

Because this plugin [passes its options](https://github.com/sindresorhus/grunt-myth/blob/2c9e60c7219561ad0fc11553d748e411a3b2b8be/tasks/myth.js#L11) onto myth, you can set myth options from it. This is useful because myth options are useful.
Good idea; PR is here: https://github.com/segmentio/myth/pull/106. I'll update this one if/when that documentation is updated.