diff --git a/signature/mechanism.go b/signature/mechanism.go
index 02ab0137..715c5a11 100644
--- a/signature/mechanism.go
+++ b/signature/mechanism.go
@@ -1,3 +1,5 @@
+// Note: Consider the API unstable until the code supports at least three different image formats or transports.
+
 package signature
 
 import (