Update and reorganize benchmarks readme to be less confusing
Update and reorganize benchmarks readme to be less confusing

We have exactly one custom runner, so the emphasis that the old
structure put on custom runners was misleading. It's better to
foreground the normal case.

lrytz/scala,lrytz/scala,lrytz/scala,scala/scala,scala/scala,lrytz/scala,scala/scala,scala/scala,scala/scala,lrytz/scala,scala/scala,lrytz/scala