Add reference to ISO 3166-1 alpha-2

[terrillmoore]

arduino-lmic/src/lmic/lmic_config_preconditions.h

Lines 74 to 78 in 16177b3

	// country codes for comparison. These values are chosen from the 2-letter domain suffixes (which
	// I think are ISO standardized)
	#define LMIC_COUNTRY_CODE_C(c1, c2) ((c1) * 256 + (c2))

1. These are standardized under ISO 3166-1 alpha-2 (see Wikipedia)
2. Should indicate that we always use lower-case by convention
3. consider C++ versions in a header file that doesn't bring all of LMIC into scope. (ArduinoLoRaWAN?)