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.
Instead of having to explain that to convert the the CLI options, would you mind doing a pull request on the Myth readme with the programmatic options?