YouTube Usage example comments (#10106)

* YouTube Usage example comments

Signed-off-by: Glenn Jocher <[email protected]>

* Update dataloaders.py

Signed-off-by: Glenn Jocher <[email protected]>

Signed-off-by: Glenn Jocher <[email protected]>

Author: glenn-jocher

utils/dataloaders.py

@@ -353,6 +353,7 @@ def __init__(self, sources='streams.txt', img_size=640, stride=32, auto=True, tr
             # Start thread to read frames from video stream
             st = f'{i + 1}/{n}: {s}... '
             if urlparse(s).hostname in ('www.youtube.com', 'youtube.com', 'youtu.be'):  # if source is YouTube video
+                # YouTube format i.e. 'https://www.youtube.com/watch?v=Zgi9g1ksQHc' or 'https://youtu.be/Zgi9g1ksQHc'
                 check_requirements(('pafy', 'youtube_dl==2020.12.2'))
                 import pafy
                 s = pafy.new(s).getbest(preftype="mp4").url  # YouTube URL