cpufreq-bench: Fix installation of bench README - enhance the README file

Currently the bench README gets overridden by the cpufrequtils README when
installing -> rename bench README to BENCH-README when installing.

The rest are bench README enhancements.

CC: ckornacker@suse.de
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>