docs: fix app.start()

README.md

@@ -465,9 +465,12 @@ state object. When calling `.toString()` instead of `.start()`, all calls to
 `send()` are disabled, and `subscriptions`, `effects` and `reducers` aren't
 loaded. See [rendering in Node](#rendering-in-node) for an in-depth guide.
 
-### tree = app.start(opts)
+### tree = app.start(rootId?, opts)
 Start the application. Returns a tree of DOM nodes that can be mounted using
-`document.body.appendChild()`. Opts can contain the following values:
+`document.body.appendChild()`. If a valid `id` selector is passed in as the
+first argument, the tree will diff against the selected node rather than be
+returned. This is useful for [rehydration](#rehydration). Opts can contain the
+following values:
 - __opts.history:__ default: `true`. Enable a `subscription` to the browser
   history API. e.g. updates the internal `state.location` state whenever the
   browser "forward" and "backward" buttons are pressed.