how to use postcss
I don't understand what this ```postcss``` does,
```javascript
if (api.config.postcss) {
        rule
          .use('postcss-loader')
          .loader('postcss-loader')
          .options(
            Object.assign(
              {
                sourceMap: !isProd
              },
              api.config.postcss
            )
          )
      }
```
