Update README.md (#11787)

Add audio format argument dependency warning

youtube_dl/options.py

@@ -751,7 +751,7 @@ def parseOpts(overrideArguments=None):
         help='Convert video files to audio-only files (requires ffmpeg or avconv and ffprobe or avprobe)')
     postproc.add_option(
         '--audio-format', metavar='FORMAT', dest='audioformat', default='best',
-        help='Specify audio format: "best", "aac", "vorbis", "mp3", "m4a", "opus", or "wav"; "%default" by default')
+        help='Specify audio format: "best", "aac", "vorbis", "mp3", "m4a", "opus", or "wav"; "%default" by default; No effect without -x')
     postproc.add_option(
         '--audio-quality', metavar='QUALITY',
         dest='audioquality', default='5',