diff --git a/site/README.md b/site/README.md
index e7ecdeb2e3..d3c1da3b07 100644
--- a/site/README.md
+++ b/site/README.md
@@ -13,7 +13,7 @@ Start the server with `npm run dev`, and navigate to [localhost:3000](http://loc
 
 ## Using a local copy of Svelte
 
-By default, the REPL will fetch the most recent version of Svelte from https://unpkg.com/svelte. To use the local copy of the compiler and runtime from this repo, you can navigate to [localhost:3000/repl?version=local](http://localhost:3000/repl?version=local).
+By default, the REPL will fetch the most recent version of Svelte from https://unpkg.com/svelte. To use the local copy of the compiler and runtime from this repo, you can navigate to [localhost:3000/repl?version=local](http://localhost:3000/repl?version=local). To produce the proper browser-compatible UMD build, you will need to run `npm run build` with the `PUBLISH` environment variable set (to any non-empty string).
 
 ## REPL GitHub integration