document `is()` method

The user of the library may not want to pass in a request object, but instead the simple media type string. Since this method is already available (albeit awkward, since the variable used in the examples is `is`), it might as well be documented. In [my use case]((https://github.com/afeld/jsonp/blob/8e6e35a703a31c0fdf0b09779e5420aeeb4c88a7/lib/jsonp.js#L12-L13)), I am trying to check the `Content-Type` of the response in Express middleware. Thanks!