This adds a new "description" field to the feed settings. This allows to
save custom description regarding a feed. It is also exported and
imported as "description" in OPML.
As per [OPML 2.0 specification]:
> Each sub-element of the body of the OPML document is a node of type rss or an outline element that contains nodes of type rss.
> Required attributes: type, text, xmlUrl.
[OPML 2.0 specification]: http://opml.org/spec2.opml#subscriptionLists