priority expander: Document random fallback
priority expander: Document random fallback

When none of the regexes match in the priorities list,
CA will fall back to a random group selection.  Document
this in the readme.
mmerrill3/autoscaler,kubernetes/autoscaler,kubernetes/autoscaler,bskiba/autoscaler,bskiba/autoscaler,kubernetes/autoscaler,mmerrill3/autoscaler,kubernetes/autoscaler,bskiba/autoscaler,mmerrill3/autoscaler,mmerrill3/autoscaler