docs: update docs for gradio

Author: soulteary

README.md

@@ -228,7 +228,6 @@ Differences may be less or more pronounced for different inputs. Please see the
 
 <img src="assets/webui.jpg">
 
-
 After you have completed the model conversion according to the above process, you can use the following command to start a simple Web UI:
 
 ```bash
@@ -253,6 +252,17 @@ To create a public link, set `share=True` in `launch()`.
 
 Open `http://0.0.0.0:7860` in your browser to start your Core ML Stable Diffusion adventure.
 
+
+Web UI relies on gradio, a great interface framework. If you have not installed it, then execute the above command, the program will try to install it automatically.
+
+If the installation fails, you can try to manually execute the following command to complete the dependency installation.
+
+```bash
+pip install gradio
+```
+
+When the installation is complete, re-execute the above command to start the Web UI.
+
 </details>
 
 ##  <a name="faq"></a> FAQ