Update README.md and example to add changeObj parameter to onChange property

[YilinGao]

I read React-CodeMirror's source code and found that the onChange property actually has 2 return values, i.e, newCode and changeObj, instead of the only one newCode in the current document. With changeObj programmers can do more elaborate work (eg, emitting event of changed values instead of the whole trunk of new code). I regard it worthwhile to document this second parameter to give programmers more choices.