Makes slightly more advanced example to show more real-world application

[mikesol]

This example helps show how subscriptions work with slightly more complicated objects and external mutations that trigger subscriptions.

[rwieruch]

Hi @mikesol Thank you so much for this contribution! I am really looking forward to merge it. In our Slack Community, I asked for help to review your PR. At the moment, I have not much time to verify that everything is working for this PR. I hope someone else steps in here. Otherwise I can review it at the end of this week. Thank you again for your work! 💯

[rwieruch]

Do you think the "Activate Subscription" section helps? Because GraphQL Playground maybe everything a user needs. Open up one tab to activate the subscription, and another tab to receive the real-time updates. Maybe we should add instructions for these steps rather than a CURL operation :)