Add on the README.md the API property useNativeAnimation

[proplayapp]

I found a compatibility issue with the Animated library and had to implicitly set the useNativeAnimation to true.
This is not mentioned on the main page, but it's already implemented in the code.

Toast.propTypes = {
style: ViewPropTypes.style,
position: PropTypes.oneOf([
'top',
'center',
'bottom',
]),
textStyle: Text.propTypes.style,
positionValue:PropTypes.number,
fadeInDuration:PropTypes.number,
fadeOutDuration:PropTypes.number,
opacity:PropTypes.number,
useNativeAnimation:PropTypes.bool
}