Can't find docs/examples of Subscriptions

### What happened?

I want to write a Subscription resolver

* Looked through this repo
* Looked through https://gqlgen.com

### What did you expect?

* Expected to find a section called "Subscriptions" explaining how to write them with gqlgen

### More info

Through some issue searching I found a link to this: https://github.com/99designs/gqlgen/blob/master/example/chat/resolvers.go#L115-L150

That's what I'm working off of currently. It would be useful to have some part of the docs touch on subscriptions.