ACPI / CPPC: Document CPPC sysfs interface

Add a file to describe the CPPC sysfs interface and steps to compute
average delivered performance using the feedback counters.

Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
[ rjw: Minor adjustments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>