cmd/bent: update very stale README.md

Also improve the sample configuration slightly.

Change-Id: Ie328f7200adbfaeca0f69214bfc5407052f0d96a
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/567201
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>