site: give example of passing a session option (#1090)

site/content/docs/04-preloading.md

@@ -39,7 +39,15 @@ So if the example above was `src/routes/blog/[slug].svelte` and the URL was `/bl
 * `page.query.foo === 'bar'`
 * `page.query.baz === true`
 
-`session` is generated on the server by the `session` option passed to `sapper.middleware` (TODO this needs further documentation. Perhaps a server API section?)
+`session` is generated on the server by the `session` option passed to `sapper.middleware`. For example:
+
+```js
+sapper.middleware({
+	session: (req, res) => ({
+		user: req.user
+	})
+})
+```
 
 
 ### Return value