Add section to the README about generic usage of interpolated value

Put is somewhere at the top.

Make sure it is clear that styles prop is not working with, well, actually styles, but gives you ability to use value to derive all other values of your styles.

See https://github.com/chenglou/react-motion/issues/304#issuecomment-331750756

React motion does not operate "style" (I agree that is a misleading name for prop). It only applies function over any number you pass. This way it can stay minimal and you have all power to interpolate any number-related values.

<Motion defaultStyle={{whatever: 0}} style={{whatever: spring(1)}}>
  {({whatever}) => (
    <div style={{color: `rgb(${255 * whatever}, ${255 * whatever}, ${255 * whatever})`}}>
      Hey, I'm transitioning color!
    </div>
  )}
</Motion>

Related:

• https://github.com/chenglou/react-motion/issues/394#issuecomment-262631976
• https://github.com/chenglou/react-motion/issues/291
• See https://github.com/chenglou/react-motion/issues/304
• ... some other closed or maybe opened issues