Add a note into README.md about missed support for IE 

As IE [doesn't support](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find#Browser_compatibility) `Object.find` function and this function is used in the code (for instance, [here](https://github.com/alexkuz/react-json-tree/blob/65b5c51dc0c8d6083ee6517c3817976e78cd843e/src/index.js#L120)), maybe it worth to mention this in the `README.md` file? 