Missing documentation for "Option 3: Express with webpack-dev-middleware (client & server)"

If you are reporting a bug or having an issue setting up React Hot Loader, please fill in below. For feature requests, feel free to remove this template entirely.

### Description

Missing documentation for "_Option 3: Express with webpack-dev-middleware (client & server)_"

### Expected behavior

Existing documentation

### Actual behavior

Missing documentation

### Environment

React Hot Loader version: `^4.2.0`
1. `node -v`: `v8.9.4`
2. `npm -v`: ` `5.6.0`

Then, specify:

1. Operating system: **Ubuntu 16.04.4 LTS xenial**
2. Browser and version: **Chrome Version 63.0.3239.132 (Official Build) (64-bit)**