sched/deadline: Add sched_dl documentation

Add in Documentation/scheduler/ some hints about the design
choices, the usage and the future possible developments of the
sched_dl scheduling class and of the SCHED_DEADLINE policy.
Reviewed-by: Henrik Austad <henrik@austad.us>
Signed-off-by: Dario Faggioli <raistlin@linux.it>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
[ Re-wrote sections 2 and 3. ]
Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1390821615-23247-1-git-send-email-juri.lelli@gmail.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/scheduler/00-INDEX

@@ -10,5 +10,7 @@ sched-nice-design.txt
 	- How and why the scheduler's nice levels are implemented.
 sched-rt-group.txt
 	- real-time group scheduling.
+sched-deadline.txt
+	- deadline scheduling.
 sched-stats.txt
 	- information on schedstats (Linux Scheduler Statistics).

kernel/sched/deadline.c

@@ -351,7 +351,8 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se,
  * disrupting the schedulability of the system. Otherwise, we should
  * refill the runtime and set the deadline a period in the future,
  * because keeping the current (absolute) deadline of the task would
- * result in breaking guarantees promised to other tasks.
+ * result in breaking guarantees promised to other tasks (refer to
+ * Documentation/scheduler/sched-deadline.txt for more informations).
  *
  * This function returns true if:
  *