__static documentation

* **electron-webpack**: 2.7.4
* **electron-builder**: 22.4.0
* **electron-updater**: N/A
* **Target**: OS X

I think there should be some additional documentation around `__static`. In particular, it should explain that:
* when you build in production mode, it points to a `static` directory inside the app's `resourcesPath` (as far as I can tell).
* if you want this to actually contain something, you'll need to do it yourself, e.g. using electron-builder's `extraResources` setting.
* this means that if you build in production mode, and then run the app with `npx electron .`, it will point to the runtime Electron app, e.g. `node_modules/electron/dist/Electron.app/Contents/Resources/static`, which will definitely never contain anything. So it really can't be used in this mode; you have to use electron-builder.
* to use it with TypeScript you'll need to add `declare const __static: string;`.