[extractor/common] Document license metafield

youtube_dl/extractor/common.py

@@ -157,6 +157,7 @@ class InfoExtractor(object):
     thumbnail:      Full URL to a video thumbnail image.
     description:    Full video description.
     uploader:       Full name of the video uploader.
+    license:        License name the video is licensed under.
     creator:        The main artist who created the video.
     release_date:   The date (YYYYMMDD) when the video was released.
     timestamp:      UNIX timestamp of the moment the video became available.