ZgatewayBT.ino: Small documentation for the different sensor signals (#222)

This PR add a small documentation to make people aware which sensor unit is going to send what kind of data.

ZgatewayBT.ino

@@ -353,7 +353,10 @@ boolean process_data(int offset, char * rest_data, char * mac_adress){
   char val[12];
   String mactopic(mac_adress);
   mactopic = subjectBTtoMQTT + mactopic;
-
+  
+ 
+  // Mi flora provides tem(perature), (earth) moi(sture), fer(tility) and lux (illuminance)
+  // Mi Jia provides tem(perature) and hum(idity)
   // following the value of digit 47 we determine the type of data we get from the sensor
   switch (rest_data[47 + offset]) {
     case '9' :