Fix doc and type

[mtgto]

1. Fix old and broken urls of webdriver.io
2. Fix sample code in README
3. Fix type of SpectronWindow

2. helps some issues like #815, #831 .

3. fixes SpectronWindow type:

import { Application } from "spectron";
let app: Application;

// ... skip setup ...

app.client.getText('#error-alert'); // compile error
const windowCount: number = app.client.getWindowCount(); // compile error
const promisedWindowCount: Promise<number> = app.client.getWindowCount();

[andersk]

I fixed this more completely in #999, using Promise<R extends PromiseLike<infer T> ? T : R> to avoid doubly-wrapped Promises, and wrapping every module in Electron, not just BrowserWindow.

[mtgto]

@andersk Great works!