Gaming Examples should include a reference to the archetype

This lets anyone following along to look at the code and run on the runner of their choice. It would probably be useful to users who are just getting started with processing unbounded datasets.

Imported from Jira [BEAM-1967](https://issues.apache.org/jira/browse/BEAM-1967). Original Jira may contain additional context.
Reported by: tgroh.

Find the Gaming Examples documentation and the relevant archetype reference; verify how users can follow the example and run it on their runner. Done means the example documentation includes that reference and supports getting started with unbounded datasets.

Written by the indexing model from the issue text.