Document -o %(upload_date)s (Closes #228)

README.md

@@ -37,8 +37,9 @@ which means you can modify it, redistribute it or use it however you like.
     -o, --output TEMPLATE    output filename template. Use %(stitle)s to get the
                              title, %(uploader)s for the uploader name,
                              %(autonumber)s to get an automatically incremented
-                             number, %(ext)s for the filename extension, and %%
-                             for a literal percent
+                             number, %(ext)s for the filename extension,
+                             %(upload_date)s for the upload date (YYYYMMDD), and
+                             %% for a literal percent
     -a, --batch-file FILE    file containing URLs to download ('-' for stdin)
     -w, --no-overwrites      do not overwrite files
     -c, --continue           resume partially downloaded files