diff --git a/README.md b/README.md
index 72e2719..c0b5f70 100644
--- a/README.md
+++ b/README.md
@@ -128,7 +128,8 @@ Start by specifying that you will be using an external configuration file:
     config-file: './.github/dependency-review-config.yml'
 ```
 
-And then create the file in the path you just specified:
+And then create the file in the path you just specified. Please note
+that the **option names in external files use underscores instead of dashes**:
 
 ```yaml
 fail_on_severity: 'critical'