added notes on using Arduino Pro Micro 3.3V with Let’s Split

keyboards/lets_split/readme.md

@@ -100,3 +100,16 @@ half to a computer by USB the keyboard will use QWERTY and Colemak when the
 right half is connected.
 
 
+Notes on Using Pro Micro 3.3V
+-----------------------------
+
+Do update the `F_CPU` parameter in `rules.mk` to `8000000` which reflects
+the frequency on the 3.3V board.
+
+Also, if the slave board is producing weird characters in certain columns,
+update the following line in `matrix.c` to the following:
+
+```
+// _delay_us(30);  // without this wait read unstable value.
+_delay_us(300);  // without this wait read unstable value.
+```