Document how to use react-hot-loader in production

[silvenon]

Fixes #1104.

[theKashey]

I am not sure --mode production does it. At least there is nothing about this in docs.

[silvenon]

It does, I tested it. Try it for yourself:

// webpack.config.js
module.exports = {
  entry: './src',
}

// src/index.js
console.log(process.env.NODE_ENV)

yarn add webpack webpack-cli
yarn webapck --mode production

// dist/index.js
// ...among bunch of other stuff...
console.log('production')

[theKashey]

👍