Provide a more beginner-friendly documentation

[MatusGasparik]

I know there is a documentation (https://github.com/vega/react-vega/tree/master/packages/react-vega) for react-vega but it's really minimal. For example it doesn't explain to pros / cons of using the different approaches, things that react-developers new to vega and vega-lite need to understand about how to use these tools. How to use vega-lite-api in react (should we?). Maybe extend it with some more tutorials that use modern React - e.g. functions-based instead of class-based components (as used in react-vega-demo).