Update of Documentation/cgroups/00-INDEX

unified-hierarchy.txt was added by 65731578 (cgroup: add documentation
about unified hierarchy)

Cc: Tejun Heo <[e-mail]>
Cc: Li Zefan <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Henrik Austad <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

Documentation/cgroups/00-INDEX

···
 	- Network priority cgroups details and usages.
 resource_counter.txt
 	- Resource Counter API.
+unified-hierarchy.txt
+	- Description the new/next cgroup interface.