add define for reserved group data type

src/helpers/TxtDataHelpers.h

@@ -6,6 +6,7 @@
 #define TXT_TYPE_PLAIN          0      // a plain text message
 #define TXT_TYPE_CLI_DATA       1      // a CLI command
 #define TXT_TYPE_SIGNED_PLAIN   2      // plain text, signed by sender
+#define DATA_TYPE_RESERVED      0x0000 // reserved for future use
 #define DATA_TYPE_DEV           0xFFFF // developer namespace for experimenting with group/channel datagrams and building apps
 
 class StrHelper {