(#50) Add comment on /example at listFiles usage

Author: LaksCastro

example/lib/screens/folder_files/folder_file_list.dart

@@ -159,7 +159,8 @@ class _FolderFileListState extends State<FolderFileList> {
       DocumentFileColumn.displayName,
       DocumentFileColumn.size,
       DocumentFileColumn.lastModified,
-      // DocumentFileColumn.id,
+      // Optional column (this can't be removed because it's required to list files)
+      DocumentFileColumn.id,
       DocumentFileColumn.mimeType,
     ];