add notice regarding timer names.

src/timers.h

@@ -27,7 +27,9 @@
 #include "cycle.h"
 #include "inline.h"
 
-/* The timers themselves. */
+/* The timers themselves. 
+   If you modify this list, be sure to change timers_names in timers.c as
+   well! */
 enum {
   timer_none = 0,
   timer_prepare,