cpuidle: Add Documentation

Documentation for cpuidle infrastructure. (resend)

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Reviewed-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX
index 8d55670..30b327a 100644
--- a/Documentation/00-INDEX
+++ b/Documentation/00-INDEX
@@ -109,6 +109,8 @@
 	- document describing CPU hotplug support in the Linux kernel.
 cpu-load.txt
 	- document describing how CPU load statistics are collected.
+cpuidle/
+	- info on CPU_IDLE, CPU idle state management subsystem.
 cpusets.txt
 	- documents the cpusets feature; assign CPUs and Mem to a set of tasks.
 cputopology.txt