cpupower: Improve cpupower build process description

Enhance cpupower build process description with the information on
building and installing the utility to the user defined directories
as well as with the information on the way of running the utility from
the custom defined installation directory.

Signed-off-by: Roman Storozhenko <romeusmeister@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>