esp32/README: Add comment about FLASH_MODE being dio.
@@ -60,6 +60,7 @@ Be sure to enter the correct path to your local copy of the IDF repository
If the Xtensa cross-compiler is not in your path you can use the
`CROSS_COMPILE` variable to set its location. Other options of interest
are `PORT` for the serial port of your esp32 module, and `FLASH_MODE`
+(which may need to be `dio` for some modules)
and `FLASH_SIZE`. See the Makefile for further information.
Building the firmware
