rel-feed was a proposal for linking to multiple (potentially alternative) h-feeds from a site's homepage, which was adding extra work & complexity to existing feed discovery methods, and has been superceded with re-using rel-alternate with
type=text/mf2+html to link to h-feed files.
The rest of this page is very much out of date and needs updating.
You should link to your separate feeds with rel=feed from your home page to:
Add the attribute
rel="feed" (or append " feed" on an existing rel attribute) on links from your homepage to your separate h-feed pages.
<link rel="feed" type="text/html" title="My blog's feed" href="feed.htm" />
How on WordPress without any coding
If you have a menu link to your main feed, you can add feed to the Link Relationship(XFN) field in the WordPress menu editor under 'Appearance' in the WordPress admin dashboard.
David Shanske's Indieweb 2016 theme automatically adds a rel-feed to your posts page in if you do not have a feed on your homepage.
Indieweb users who publish rel=feed on their site
Kartik Prabhu uses rel=feed on kartikprabhu.com since at least 2015-01-01.
bear uses rel=feed on bear.im since at least 2015-08-01 to link to his feeds page from his front page.
Pelle Wessman uses rel=feed on voxpelli.com since 2015-10-19 to link to his feeds for bookmarks and social interactions.
People/code that consumes rel=feed
Parse This is a WordPress library that parses web pages into jf2 for use in the Post Kinds and Yarns Microsub plugins. It looks for rel-feed.