Had a project recently where I need to output the RSS feed url of the current taxonomy archive page. The function to use is
get_term_feed_link ( $term_id, $taxonomy = 'category', $feed = '' ). Read more at WPSeek.
This function is yet to be documented in the Codex, but it is quite similar with
get_category_feed_link() function, which is an identical function, but only works with categories.