message: Update README.md
  add live demo link
diff: [1.x Docs](https://ustbhuangyi.github.io/better-scroll/#/)
  [2.x Docs](https://better-scroll.github.io/docs/en-US/)
  +[2.x Demo](https://better-scroll.github.io/examples/)
  > **Note**: The following code is the syntax of `2.x`.
  