Add documentation.

README.adoc

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+= download-yt-feeds
+:url-ytdl-format: https://github.com/ytdl-org/youtube-dl/#format-selection
+
+Download YouTube-feeds like they are podcasts. Files will be put in `<download_dir>/channel/title.extension`.
+
+== Configuration
+
+The configuration directory is `${XDG_CONFIG_HOME}/download-yt-feeds` or, if
+`${XDG_CONFIG_HOME}` is not set, `${HOME}/.config/download-yt-feeds`. You need 2
+files in this directory, `config` and `feedlist`.
+
+=== config
+
+.Example configuration file
+[source,shell]
+--------------------------------------------------------------------------------
+# This file will be source-d.
+download_dir="/home/user/Podcasts/youtube"
+keep_for_days="14"
+ytdl_format="bestvideo[height<=1080]+bestaudio/best[height<=1080]"
+--------------------------------------------------------------------------------
+
+|===============================================================================
+| *download_dir*  | The directory to which the videos will be downloaded.
+| *keep_for_days* | Delete videos after this many days.
+| *ytdl_format*   | Same syntax as link:{url-ytdl-format}[youtube-dl --format]. Default is "best".
+|===============================================================================
+
+
+=== feedlist
+
+.Example feed list
+[source,cfg]
+--------------------------------------------------------------------------------
+# Everything that doesn't start with "http" will be skipped.
+https://www.youtube.com/feeds/videos.xml?channel_id=UCyczvB_IvA7V1a5BXXu3CRg # BlackRock TV
+https://www.youtube.com/feeds/videos.xml?user=lefticus1 # C++ Weekly
+--------------------------------------------------------------------------------