doc: add note about naming of filename

Author: vidhill

readme.md

@@ -48,6 +48,9 @@ It requires testify >= v1.7.0
 
 Mocktail can only generate mock of interfaces inside a module itself (not from stdlib or dependencies)
 
+The `// mocktail` comments **must** be added to ae file named `mock_test.go` only,  
+comments in other files will not be detected
+
 ## Examples
 
 ```go