docs: default value for mapping

docs/advanced/custom_mapping.md

@@ -1,5 +1,7 @@
 We can use this feature to map each field with meaningful value to display in the table. For example, the category field contains 1, 2, and 4 values, but when those values are displayed, the user might get confused as those values do not signify the meaning of their mapping. To avoid this confusion, the user can map each field with meaningful value as shown in the following example:
 
+If you have fields that are not mandatory but you would like to display them inside table, you can use default value option by providing ```"[[default]]"``` as one of parameters (check example bellow). It is a way to provide some meaningful information instead of displaying empty field.
+
 ### Usage
 
 ```json
@@ -22,7 +24,8 @@ We can use this feature to map each field with meaningful value to display in th
                 "mapping": {
                     "1": "Global",
                     "2": "US Gov",
-                    "4": "China"
+                    "4": "China",
+                    "[[default]]": "Unknown"
                 }
             }
         ],