README: Add brief description for manpages/apropos.

Trying to mimic the manpage of (GNU) `ls`, we don't conjugate the verb as
"downloads" or something else.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>

README.md

@@ -1,7 +1,7 @@
 % YOUTUBE-DL(1)
 
 # NAME
-youtube-dl
+youtube-dl - download videos from youtube.com or other video platforms
 
 # SYNOPSIS
 **youtube-dl** [OPTIONS] URL [URL...]