Docs: update README with a more idiomatic example

[eledhwen]

The README example has the merit of being easy to grasp, but could be made more useful by showcasing multi-results + aggregation patterns that are more in line with "why" someone could benefit from it.

We could do a similar pipeline with a setup such that:

• several steps produce the same type of result
• one of them produce a collection of such result
• a sink consumes said results as a Stream