Add a tip for testing custom 404.html on hugo server

- https://github.com/gohugoio/hugo/issues/874
- https://discourse.gohugo.io/t/how-to-test-404-html-on-local-server/681/2

content/templates/404.md

@@ -52,4 +52,10 @@ Your 404.html file can be set to load automatically when a visitor enters a mist
 * Amazon AWS S3. When setting a bucket up for static web serving, you can specify the error file from within the S3 GUI.
 * Caddy Server. Using `errors { 404 /404.html }`. [Details here](https://caddyserver.com/docs/errors)
 
-[pagevars]: /variables/page/
+{{% note %}}
+`hugo server` will not automatically load your custom `404.html` file, but you
+can test the appearance of your custom "not found" page by navigating your
+browser to `/404.html`.
+{{% /note %}}
+
+[pagevars]: /variables/page/