Added getCurrentOrder method and documentation

[kernel-sanders]

Closes #99

[jcurtis]

Are you not able to use onChangeOrder and keep the order in state?

[jcurtis]

Ah I see you're having issue with performance using the state. If you need to keep the order but your render doesn't actually depend on it, you can keep in an instance variable instead. That way your component won't be forced to re-render.

class Foo extends React.Component { order = []; render() { return <SortableList onChangeOrder={newOrder => this.order = newOrder} /> } } 

[jcurtis]

The React docs explain why it's not a good idea to use refs for this type of logic https://reactjs.org/docs/refs-and-the-dom.html

[kernel-sanders]

Using an instance variable accomplishes my needs without hurting performance and without using a ref! Thanks for the education!