docs(quadchecker): require solution to be submitted in one file

subjects/quadchecker/README.md

@@ -12,7 +12,14 @@ Create a program `quadchecker` that takes a `string` as an argument and displays
 
 - If there is more than one `quad` matches, the program must display them all alphabetically ordered and separated by a `||`.
 
-Make sure you submit all the necessary files to run the program.
+The solution should be submitted as follow:
+
+```console
+$ tree .
+quadchecker
+├── go.mod
+└── main.go
+```
 
 ### Usage