Add how to convert video section in README.md

Close https://github.com/wkentaro/labelme/issues/427

examples/video_annotation/README.md

@@ -15,3 +15,17 @@ labelme data_annotated --labels labels.txt --nodata --keep-prev
 <img src=".readme/data_annotated.gif" width="98%" />
 
 *Fig 2. Visualization of video semantic segmentation.*
+
+
+## How to Convert a Video File to Images for Annotation?
+
+```bash
+# Download and install software for converting a video file (MP4) to images
+wget https://raw.githubusercontent.com/wkentaro/dotfiles/f3c5ad1f47834818d4f123c36ed59a5943709518/local/bin/video_to_images
+pip install imageio imageio-ffmpeg tqdm
+
+python video_to_images your_video.mp4  # this creates your_video/ directory
+ls your_video/
+
+labelme your_video/
+```