Add documentation to use with other Reactive Stream implementations

See https://github.com/apache/pulsar-client-reactive/issues/10#issuecomment-1293755039

> One possible solution to the problem is to use Project Reactor's .as method (`

P as(Function, 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

Read the README and the referenced comment in issue #10 first. Add a tips section documenting how Project Reactor's .as method can convert Flux or Mono to other Reactive Streams implementations, including the RxJava3 examples given in the issue; the README should clearly explain these usage options.

Written by the indexing model from the issue text.