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.

```jsx

{({whatever}) => (

```

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

Open the README and review the linked discussion about the styles prop and generic interpolation. Add a section near the top explaining that styles carries values used to derive actual styles, including the provided color-transition example. Done means the README clearly communicates this usage without implying that styles itself accepts CSS styles.

Written by the indexing model from the issue text.