diff --git a/lib-extra/README.md b/lib-extra/README.md
index 5247029ab..a2fae3a17 100644
--- a/lib-extra/README.md
+++ b/lib-extra/README.md
@@ -22,6 +22,7 @@ e.g. For an Erlang plugin named `plugin_foo`:
 ## Build a release
 
 ```
+$ export EMQX_EXTRA_PLUGINS=plugin_foo,plugin_bar
 $ make
 ```