diff --git a/wordpress/README.md b/wordpress/README.md
index bd9bcc178..2b4d24c5b 100644
--- a/wordpress/README.md
+++ b/wordpress/README.md
@@ -84,6 +84,7 @@ The following environment variables are also honored for configuring your WordPr
 -	`-e WORDPRESS_TABLE_PREFIX=...` (defaults to "", only set this when you need to override the default table prefix in wp-config.php)
 -	`-e WORDPRESS_AUTH_KEY=...`, `-e WORDPRESS_SECURE_AUTH_KEY=...`, `-e WORDPRESS_LOGGED_IN_KEY=...`, `-e WORDPRESS_NONCE_KEY=...`, `-e WORDPRESS_AUTH_SALT=...`, `-e WORDPRESS_SECURE_AUTH_SALT=...`, `-e WORDPRESS_LOGGED_IN_SALT=...`, `-e WORDPRESS_NONCE_SALT=...` (default to unique random SHA1s)
 -	`-e WORDPRESS_DEBUG=1` (defaults to disabled, non-empty value will enable `WP_DEBUG` in `wp-config.php`)
+-	`-e WORDPRESS_CONFIG_EXTRA=...` (defaults to nothing, non-empty value will be embedded verbatim inside `wp-config.php` -- especially useful for applying extra configuration values this image does not provide by default such as `WP_ALLOW_MULTISITE`; see [docker-library/wordpress#142](https://github.com/docker-library/wordpress/pull/142) for more details)
 
 If the `WORDPRESS_DB_NAME` specified does not already exist on the given MySQL server, it will be created automatically upon startup of the `wordpress` container, provided that the `WORDPRESS_DB_USER` specified has the necessary permissions to create it.