fix readme

README.md

@@ -268,4 +268,11 @@ pip install git+ssh://git@git.wanmeizhensuo.com/wushengyu/maskrcnn.git
 from demo.three_color import handle_from_path
 path = '/xxx/xxx/xx.jpg'
 img = handle_from_path(path)        # 类似cv2.imread的img
+
+
+from demo.three_color import handle_from_url
+url = 'http://xxx/xxx/xx.jpg'
+img = handle_from_url(url) 
+
+
 ```

demo/three_color.py

@@ -64,7 +64,7 @@ def img_to_three_color_map(img):
         min_image_size=args.min_image_size,
     )
     composite = coco_demo.run_on_opencv_image(img)
-    if composite:
+    if composite is not None:
         return composite
     return None