Add method for querying service name

Author: stefan-sherwood

Diff for: libraries/ESPmDNS/src/ESPmDNS.h

@@ -110,6 +110,7 @@ class MDNSResponder {
   }
 
   String hostname(int idx);
+  String instanceName(int idx);
   IPAddress address(int idx);
   IPAddress addressV6(int idx);
   uint16_t port(int idx);