diff --git a/README.md b/README.md
index a353685c87b..306a110db6c 100644
--- a/README.md
+++ b/README.md
@@ -303,6 +303,9 @@ Support this project with a :star:
 > **Warning**
 >
 > *   When using `files_yaml*` inputs ensure all outputs are prefixed by the key `test_{...}` e.g. `test_added_files`, `test_any_changed`
+> *   All keys must start with a letter or _ and contain only alphanumeric characters, -, or _.
+
+
 
 ## Outputs 📤