version.sh: add note that ffversion.h is auto-generated

Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

version.sh

@@ -54,6 +54,7 @@ GUARD=$(echo "$2" | sed 's/\//_/' | sed 's/\./_/' | tr '[:lower:]' '[:upper:]' |
 # Update version header only on revision changes to avoid spurious rebuilds
 if test "$NEW_REVISION" != "$OLD_REVISION"; then
     cat << EOF > "$2"
+/* Automatically generated by version.sh, do not manually edit! */
 #ifndef $GUARD
 #define $GUARD
 $NEW_REVISION