[docs] Add example of multiple parallel listeners

[mecampbellsoup]

Hello guys! New to the project and golang to some extent.

I found the existing listen example very helpful. I wanted to ask if you would consider adding an example that is more like "real life" in which an application is listening to multiple distinct channels, and how one might go about writing the supporting Go code in a way that is idiomatic and "correct" 😅

[johto]

Is my answer in #1139 helpful?