diff --git a/README.md b/README.md
index f15a8fb..ce70c7a 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,15 @@ Documentation is at https://periph.io
 [![Gitter chat](https://badges.gitter.im/google/periph.png)](https://gitter.im/periph-io/Lobby)
 
 
+## WARNING
+
+**This library is unstable. Breaking API changes occur frequently.**
+
+If you want to use `periph.io/x/periph`, please [vendor
+periph](https://github.com/golang/go/wiki/PackageManagementTools) first and
+frequently roll.
+
+
 ## Example
 
 ~~~go