Remove "hmr" option from Webpack docs (#846)
Remove "hmr" option from Webpack docs (#846)

After upgrading Webpack, I was getting this error:

```
ValidationError: Invalid options object. Mini CSS Extract Plugin Loader has been initialized using an options object that does not match the API schema.
 - options has an unknown property 'hmr'. These properties are valid:
   object { publicPath?, emit?, esModule?, layer? }
```

Seems like this option isn't supported any more.
callstack/linaria,callstack/linaria,callstack/linaria