Add a MESSAGE to document that further tools can be needed for some sites
(but aren't strictly needed for most supported sites).

Thanks to coypu who pointed out that via a private email.


(leot)

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$NetBSD: MESSAGE,v 1.1 2016/03/11 18:39:05 leot Exp $

youtube-dl works fine for most supported sites.

For some extractors youtube-dl also needs
multimedia/{ffmpeg,ffmpeg2,ffmpeg3}, net/rtmpdump and multimedia/mplayer or
multimedia/mpv. All these packages are not automatically installed and it
is needed to manually install them if required.
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