Add documentation for scopes?

Took a bit to figure out how to change these settings when running the OAuth request; actually even PHP League's base package doesn't mention this. I'll reach out them as well, but I had to look at other libraries to find the method: https://github.com/thephpleague/oauth2-linkedin#managing-scopes

```
$options = [
'scope' => ['abc','xyz'] // array or string
];

$authorizationUrl = $provider->getAuthorizationUrl($options);
```

Can be pretty important depending on what scopes you need to use with the OAuth + Stripe.