Give your starting passage the tag "start", then put this somewhere on it:
(if: (saved-games:) contains "auto")[(link: "Continue")[(load-game: "auto")]]
Then create a passage, give it the tag "footer" and put this into it:
(if: not ((passage:)'s tags contains "start"))[(save-game: "auto")]
Now there will be a link on your starting passage that will allow your players to continue their last game, as long as their browser allowed a save to be created.
WARNING: The more passages the Reader visits the large the Story History and each Auto-Save can become, which in turn means it can take slightly longer each time to do the save. This can eventually lead to a noticeable delay each time the Reader changes passages.