Exemplify realistic subscriptions

[kaqqao]

We need an example showcasing subscriptions over websockets. Probably the best way would be to port @bbakerman's example from graphql-java.

Another interesting example would be to integrate with Spring's own event system (via ApplicationEventPublisher).

There's one existing port of @bbakerman's example written by @estebes here: https://github.com/estebes/graphql-subscriptions-websockets