Added warning for capital field names in structs

Names without capital letters will be ignored by the JSON Marshaller. This may cause confusion when fields are not being unmarshalled correctly without a warning why.

examples/json/json.go

@@ -14,6 +14,8 @@ type response1 struct {
     Page   int
     Fruits []string
 }
+// Fields must have capital letters to be exposed to other
+// programs such as the JSON Marshaller. 
 type response2 struct {
     Page   int      `json:"page"`
     Fruits []string `json:"fruits"`