Suggestion: Simplify example

[derwaldgeist]

At first glance, the example provided kinda turned me off, because it is overly complicated. It includes the (rather rare) case that a subscription adds a field to each document that is sub-specific. I'd like to recommend to split this example into two: One for the super-simple basic behavior, and one with this edge case.

[mitar]

PR welcome. :-)