Update app.py

Add an extended description

app.py

@@ -72,6 +72,11 @@ control_task_list = [
 ]
 with block:
     gr.Markdown("## Adding Conditional Control to Text-to-Image Diffusion Models")
+    gr.HTML('''
+     <p style="margin-bottom: 10px; font-size: 94%">
+                This is a simple demo for ControlNet, which is a neural network structure to control diffusion models by adding extra conditions such as canny edge detection. The demo is based on the <a href="https://github.com/lllyasviel/ControlNet" style="text-decoration: underline;" target="_blank"> Github </a> implementation. 
+              </p>
+              ''')
     with gr.Row():
         with gr.Column():
             input_image = gr.Image(source='upload', type="numpy")