suggest always passing `finish` step in README?

[vtjnash]

This seems like the intended behavior of parallel-finished. It means parallel-finished might run multiple times if the user explicitly re-runs failed jobs, but means partial coverage will get updated once all of the CI runs end (even if few or none of them succeeded and uploaded coverage).

I wasn't certain if this should suggest only doing this for always() && github.event_name == 'pull_request', or some other combination of options, depending on how the diff-changed algorithm works across partially-failed builds. But this seemed like a simple enough improvement regardless.