cgroup: update Documentation/cgroups/00-INDEX

There are new files added to cgroup documentation.  Let's update the
index file to include the new files.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>