Add README and update pm-graph and sleepgraph docs

Config/man page/README files:
 - include README in the pm-graph folder
 - add more detail to the example config to describe more options
 - update the sleepgraph man page to document the new arguments

Signed-off-by: Todd Brandt <todd.e.brandt@linux.intel.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>