Readme has incorrect info re: API keys

Hi! Hope you're all doing well after these two difficult years. Just want to note a small possible issue I found with the readme here.

The readme notes that this library offers `integration with the googleauth gem for authentication using OAuth, API keys, and service accounts`. However, after much experimentation, it seems like the (only) way to use an API key is directly through the service -- for instance

```ruby
service = Google::Apis::YoutubeV3::YouTubeService.new
service.key = my_api_key
```

Just want to suggest updating the readme to make that clear for future developers looking to make API calls to resources that support API key authorization. (Might also be worth making a note about the need to include a `referer` and configure that on the API key, since that took some time to figure out as well.)

Thanks!