[ServiceWorker] Add example for Service-Worker-Allowed (#775)

I imagine a lot of people will want to do this: 

```JS
navigator.serviceWorker.register("/js/sw.js", {scope: "./"});
```
Because having a sw.js sitting in the root is just ugly. 

However, the `Service-Worker-Allowed` header is mega confusing to use, and the corresponding spec text is (for me?) very hard to understand. 

Can we please add an example for how to use this in the common case above... also, can we please add some examples for how the `{scope: "whatever"}` and the `Service-Worker-Allowed` header value are combined to come up with `maxScope`, etc.?

Thanks! :heart:
 

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/775