helix serial.c: add some comment

diff --git a/keyboards/helix/serial.c b/keyboards/helix/serial.c
index c813b6b9ae0dbb5856322e1f679c8b3cda1cdcd7..24b889d30b28a5a56008b19c62768041d8ddd08b 100644 (file)
--- a/keyboards/helix/serial.c
+++ b/keyboards/helix/serial.c
@@ -1,5 +1,10 @@
 /*
  * WARNING: be careful changing this code, it is very timing dependent
+ *
+ * 2018-10-28 checked
+ *  avr-gcc 4.9.2
+ *  avr-gcc 5.4.0
+ *  avr-gcc 7.3.0
  */
 
 #ifndef F_CPU