diff --git a/README.md b/README.md
index 0ff75682..367d99a8 100644
--- a/README.md
+++ b/README.md
@@ -124,4 +124,5 @@ sudo pip install . && coverage run tests/test.py
 coverage combine && coverage report -m
 ```
 
-To read events, `evtest` is very helpful.
+To read events, `evtest` is very helpful. Add `-d` to `key-mapper-gtk`
+to get debug outbut.