Add undocumented prop to README

[facuacostag]

I was checking the code for the dropdown because it was rendering very slow in my app with just 3 items and I realised that there is a prop not documented that activates some sort of delay. This prop is rippleDuration and if it's set to 0 the animation improves a lot.

let {
      data,
      valueExtractor,
      onChangeText,
      animationDuration,
      rippleDuration,
    } = this.props;