[RN2483] Add some example code for new RN2483 library

Since I have rewritten the entire library, I may also add some short example code to illustrate the new async features.

lib/RN2483-Arduino-Library/src/rn2xx3_handler.cpp

@@ -456,6 +456,10 @@ void rn2xx3_handler::setAsyncMode(bool enabled) {
   _asyncMode = enabled;
 }
 
+bool rn2xx3_handler::getAsyncMode() const {
+  return _asyncMode;
+}
+
 bool rn2xx3_handler::useOTAA() const {
   return _otaa;
 }