Add clarifying comment in KeyboardLayout.h

This library only supports ASCII characters, even with international
layouts. Although this was already implicit in the sentence “Layout
files map ASCII character codes to keyboard scan codes”, we may as well
say explicitly that non-ASCII characters are not supported.

Author: edgar-bonet

src/KeyboardLayout.h

@@ -5,6 +5,8 @@
   only in Keyboard.cpp and the keyboard layout files. Layout files map
   ASCII character codes to keyboard scan codes (technically, to USB HID
   Usage codes), possibly altered by the SHIFT or ALT_GR modifiers.
+  Non-ACSII characters (anything outside the 7-bit range NUL..DEL) are
+  not supported.
 
   == Creating your own layout ==