Describe how to use (or integrate) results reporter in test runner

**Context**
Test runner currently saves logs to the artifacts folder. (since https://github.com/webbertakken/unity-test-runner/pull/37) That way we can use test results reporter (which generates a html report)

**Solution**
Either we have to add a section to the readme, about how to run an additional action for producing the html report, or we should integrate the reporter in this action and support it natively. Generating a report doesn't sound too heavy to me.

What are your thoughts?
Do we know how longit takes for the report to generate?

Start with the README and the test runner's existing artifact/log output described in the issue, then check how the test results reporter consumes those artifacts to generate HTML. Done means either a documented additional action workflow or a clearly scoped native integration, including its report-generation behavior and timing.

Written by the indexing model from the issue text.