Add warning about the importance of using the older Arduino

Prog_SWD_D11C/README.md

@@ -118,6 +118,8 @@ Verification.... done.
 
 Now, you can program the hello board using Arduino. You can disconnect the programmer now if you want. 
 
+> **Important**: Use the [1.8.X](https://www.arduino.cc/en/software#legacy-ide-18x) version of the Arduino IDE (the older one). At the moment of writing this the recommended version is 1.8.19.
+
 In Arduino, you will need to install additional board definitions. Refer to [this tutorial](https://gitlab.cba.mit.edu/quentinbolsee/Arduino-SAM-tutorial/) in order to get them installed.
 
 As a result you should see the ´Fab SAM core for Arduino´ listed under "Tools > Boards" in Arduino IDE. You should also see a serial port available under "Tools > Port".

Prog_UPDI_D11C/README.md

@@ -67,6 +67,8 @@ CPU reset.
 
 At this point you will need to instal the [megaTinyCore](https://github.com/SpenceKonde/megaTinyCore) Arduino board definitions. 
 
+> **Important**: Use the [1.8.X](https://www.arduino.cc/en/software#legacy-ide-18x) version of the Arduino IDE (the older one). At the moment of writing this the recommended version is 1.8.19.
+
 As a result, you should be able to see the ATtiny boards under "Tools > Board > megaTinyCore".
 
 For the sake of this example, we are going to program the [Hello ATtiny412](https://gitlab.fabcloud.org/pub/programmers/hello-attiny412) board.