added short explanation readme to nginx example

examples/nginx_subpath/README.md

@@ -0,0 +1,14 @@
+# Serving app behind reverse proxy subpath
+
+This demo shows how to serve NiceGUI on a subpath behind a reverse proxy.
+Here we use Nginx. For a Traeffic demo have a look at https://github.com/zauberzeug/nicegui/blob/main/docker-compose.yml.
+
+## Try Out
+
+Just run
+
+```bash
+docker-compose up
+```
+
+Then you can access http://localhost/nicegui.