This app shouldn't need to exist.
If you visit a site with an RSS feed and tap the Feed Hawk extension, Feed Hawk will subscribe to the feed and display a confirming alert.
RSS apps should have an extension built in. None of them do, so Feed Hawk was born. I feel silly paying $3 for it, but I feel sillier using a clunky bookmarklet to subscribe to feeds, and not being able to subscribe at all in Safari View Controller.