doc: add missing files to timers/00-INDEX

- timers-howto was added by commit 0fcb8081 (Documentation: Add
  timers/timers-howto.txt)
- NO_HZ was added by commit 0c87f9b5 (nohz_full: Add documentation.)

Cc: Patrick Pannuto <ppannuto@codeaurora.org>
Signed-off-by: Henrik Austad <haustad@cisco.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/timers/00-INDEX

@@ -8,5 +8,9 @@ hpet_example.c
 	- sample hpet timer test program
 hrtimers.txt
 	- subsystem for high-resolution kernel timers
+NO_HZ.txt
+	- Summary of the different methods for the scheduler clock-interrupts management.
+timers-howto.txt
+	- how to insert delays in the kernel the right (tm) way.
 timer_stats.txt
 	- timer usage statistics