Move sched-rt-group.txt to scheduler/

Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/scheduler/00-INDEX

@@ -12,5 +12,7 @@ sched-domains.txt
 	- information on scheduling domains.
 sched-nice-design.txt
 	- How and why the scheduler's nice levels are implemented.
+sched-rt-group.txt
+	- real-time group scheduling.
 sched-stats.txt
 	- information on schedstats (Linux Scheduler Statistics).