add info on how to debug irq handler issues..

NOTES.md

@@ -76,4 +76,15 @@ monitor reset halt
 will reset the device, and halt it immediately, allowing setting a break
 point for main, or earlier.
 
+Weird Handlers
+--------------
+
+When using gdb you might end up in a weird handler that is unexpected.  This is
+because the DefaultHandler will not always be obvious, use the following command
+to generate Handlers for all the weak ones to figure out which is the correct
+handler being hit:
+```
+arm-none-eabi-nm build/rs485gw.elf  | awk '$2 == "W" && && $3 != "Reset_Handler" && $3 ~ /Handler$/ { print "void " $3  "(){ for(;;); }" }'
+```
+
 [1] https://heltec-automation-docs.readthedocs.io/en/latest/stm32/lora_node_151/pingpong_test.html