[README] Mention _og_search_description (#4304)

Lots of sites do have this meta tag, so just add it to the example.

README.md

@@ -500,6 +500,7 @@ ### Adding support for a new site
             return {
                 'id': video_id,
                 'title': title,
+                'description': self._og_search_description(webpage),
                 # TODO more properties (see youtube_dl/extractor/common.py)
             }
     ```