diff --git a/README.md b/README.md
index afbfad3..e06dcd9 100644
--- a/README.md
+++ b/README.md
@@ -51,7 +51,6 @@ render(
 );
 ```
 
-
 ----------
 
 Alternatively, you can load it directly from the [UNPKG][unpkg] CDN using vanilla HTML and JavaScript:
@@ -72,6 +71,8 @@ history.init().then(function () {
 });
 ```
 
+(`app-history` is built for ES5 but require support for `Promise` objects. To ensure compatibility you may want to load a Promise polyfill too.)
+
 ## Usage
 
 ### Going back