Add documentation to use with other Reactive Stream implementations

[cbornet]

See #10 (comment)

One possible solution to the problem is to use Project Reactor's .as method (<P> P as(Function<? super Flux<T>, P> transformer)) to convert a Flux or Mono to another type.
For example, converting to RxJava3, would be as simple as .as(Flowable::from) or .as(Single::from).
Similar chaining with .as method could be done for Akka Streams and any other Reactive Streams implementation.

We should add doc for this in a tips section of the README