Improve README.md

[mifi]

Fix multiple docs issues:

• Example code doesn't work, and it doesn't set the session in the callback, because when a custom callback is passed as the second argument to passport.authenticate('apple', callback), the whole internal logic inside passport that calls req.logIn gets bypassed, see https://github.com/jaredhanson/passport/blob/217018dbc46dcd4118dd6f2c60c8d97010c587f8/lib/middleware/authenticate.js#L221
• The error handling in the callback did not work.
• Document and work around problems with SameSite browser restrictions.
• Remove broken demo link.

[ananay]

Thanks so much, the README definitely needed an update. I'll add the example link back once I have it back up.