Fails in IE

[feychou]

Since Promise is not polyfilled, the animation won't work on IE (till latest; it works on Edge though).

This will cause all hosting apps that rely on react-state-animation and have no polyfill installed to get the same error on IE.

[feychou]

I am not sure what the best way to go about it is, many libs just have a warning in the README about compatibility and let the hosting application install the polyfills. That may be best practice in terms of bundle size.

Would you accept a PR with a warning in the README and a polyfill installed in the demo?

[tejitak]

Thanks for opening the issue.
I appreciate if you create a PR which contains the README updates and polyfill installation for the demo code.