message: docs(install): add next install tag
diff: BetterScroll is hosted on NPM and executed with the following command:
  ```bash
  -npm install @better-scroll/core --save
  +npm install @better-scroll/core@next --save
  // or
  -yarn add @better-scroll/core
  +yarn add @better-scroll/core@next
  ```
  The next step is to use it in the code. [webpack](https://webpack.js.org/) and other build tools support the introduction of code from `node_modules` :
  