Add debugging docs

DEBUGGING.md

@@ -0,0 +1,17 @@
+# Debugging
+
+When the ESP crashes and it's connected to the serial port in debug mode, you can get its stacktrace.
+
+But then you need to decode it to see what's going on. To do that you need to:
+
+- install https://github.com/janLo/EspArduinoExceptionDecoder/
+
+- save the stacktrace in a file, e.g. debug.txt
+
+- run `python3 ~/.platformio/packages/toolchain-xtensa/decoder.py -e .pio/build/debug/firmware.elf debug.txt -s`
+
+References:
+- https://github.com/esp8266/Arduino/blob/master/doc/faq/a02-my-esp-crashes.rst
+- https://arduino-esp8266.readthedocs.io/en/latest/exception_causes.html
+- https://arduino-esp8266.readthedocs.io/en/latest/Troubleshooting/stack_dump.html
+