Reddit allow subreddits to have a wiki in addition to the submission feed. These can't be easily accessed on Red Reader, unless the sidebar, a post or comment contain an explicit link to a wiki page.

As of now, following a link to a subreddit's wiki opens the link in the internal browser:

For reference, the /api/info returns a wiki_enabled value when a t5 (subreddit) object is passed as a param. There are also multiple wiki-specific API endpoints, of which [/r/subreddit]/wiki/page and possibly [/r/subreddit]/wiki/pages are useful for reading wiki pages.

A possible approach would be to add a "Wiki" menu item for subreddits where wiki_enabled==true, linking to the subreddit's wiki index, to be opened in an HtmlViewActivity.