[doc] add open-webui example

Signed-off-by: reidliu41 <reid201711@gmail.com>

Author: reidliu41

examples/online_serving/open_webui/open_webui.md

@@ -0,0 +1,25 @@
+# Setup Open WebUI with vLLM
+
+1. Install the (Docker)[https://docs.docker.com/engine/install/]
+
+2. Start the vLLM server with supported chat completion, e.g.
+
+```console
+vllm serve qwen/Qwen1.5-0.5B-Chat
+```
+
+1. Start the (Open WebUI)[https://github.com/open-webui/open-webui] docker container (replace the vllm serve host and vllm serve port):
+
+```console
+docker run -d -p 3000:8080 \
+--name open-webui \
+-v open-webui:/app/backend/data \
+-e OPENAI_API_BASE_URL=http://<vllm serve host>:<vllm serve port>/v1 \
+--restart always \
+ghcr.io/open-webui/open-webui:main
+```
+
+1. Open it in the browser: <http://open-webui-host:3000/>
+
+On the top of the web page, you can see the model `qwen/Qwen1.5-0.5B-Chat`.
+![Spans details](https://imgur.com/a/pm1VRqG)