update readme

Signed-off-by: junjiejiangjjj <junjie.jiang@zilliz.com>

README.md

@@ -53,6 +53,17 @@ av
 
 
 ## How it works
+```python
+from towhee.dc2 import pipe, ops, DataCollection
+
+p = (
+    pipe.input('video_file')
+    .flat_map('video_file', 'frame', ops.video_decode.ffmpeg(start_time=10.0, end_time=15.0, sample_type='time_step_sample', args={'time_step': 1}))
+    .output('frame')
+)
+
+DataCollection(p('./video.mp4')).show(limit=1)
+```
 
 ```python
 from towhee import ops