Add a note about providing a middleware to set a user session

[danawoodman]

For a new user it might not be clear that they'll need to provide req.user via a middleware, so I've added a line to make this explicit that they'll need to create some sort of middleware to set the user for the sapper.middleware

Relates to #1437

[benmccann]

Thanks. It might help some, but I'm afraid it also sort of raises the question of how to create userMiddleware. I don't really want to spend much more time on the Sapper docs, so maybe it's better if we just make sure the SvelteKit docs are sufficiently clear on this point

[danawoodman]

Ok, up to you. Now that it is in Beta it makes sense to throw energy at it instead 👍

[benmccann]

Thanks!