[Suggestion]Make the doc more detailed

[NE-SmallTown]

Thanks for this repo,it is really very very nice!But there maybe are a few things to make it better.

1.just as #23 says,we need to remove the unknown props in html tag because react doesn't allow us to do this,if we need,we can add the data- prefix or wait the react 16 release(it will allow to you to make custom prop on html tag).

2.the setup says:

// (optional)
// We can wrap any component using the factory methods
// Assume that `MyClass` is a React class we created
SparkScroll['MyClass'] = sparkScrollFactory(MyClass);

<SparkScroll.MyClass
   myClassProperty="some value that MyClass requires"
     timeline={{
     'topTop+100': {width: '0%', backgroundColor: '#5c832f'},
     'topTop+250': {width: ['100%', 'easeOutQuart'], backgroundColor: '#382513'}
}} />

but there is no demo for it and the usage above probably is wrong because it will cause sparkAnimator.update is not a function error because we don't set the animator in the options by check the src code

3.Finally,maybe we could also add something about SparkProxy and tell user we could use negative number of the timeline option's property in the doc.

@gilbox Thanks again for creating this repo! :+1: