Add section "Serial port" telling the user to set FLASHING_SERIAL_PORT variable in case they use stm32flash.

Author: DarinelSanguino

guides/flashing.md

@@ -20,6 +20,12 @@ There are differennt interfaces (serial, I2C, SPI, SWD, etc.) and programs you c
 
  - [st-flash](https://github.com/stlink-org/stlink) uses SWD interface. (default)
  - [stm32flash](https://github.com/stm32duino/stm32flash) uses serial interface.
+ 
+### Serial port
+
+Follow this step only if you are using `stm32flash`.
+
+In your makefile set `FLASHING_SERIAL_PORT` variable to the name of the serial port to which your device is connected.
 
 ### Connect an LED
 
@@ -49,6 +55,7 @@ Before flashing your device, make sure to have done all the things on this check
 
  - Make sure to change the `DEVICE` variable value in the make file to the name of your device;
  - Make sure to set the `FLASHING_TOOL` variable to the name of your flashing tool;
+ - If you are using `stm32flash`, make sure to set the `FLASHING_SERIAL_PORT` variable to the name of your serial port;
  - Make sure to have changed the pin to "blink" if the LED on your development board is not connected to pin `PC13`;
  - Optionally, make sure to connect an LED to your development board if your development board does not have one built-in;
  - Make sure that you have properly connected the debugger to your development board;