Add note about "secure contexts" for offline mode

[rozbb]

Currently, if you run scripts/prod.sh on your local network, offline mode will fail. The reason for this is because service workers can only be registered in "secure contexts" meaning localhost or http://..., and no more. This might be surprising for a developer trying out the server for the first time and finding that one of the core features doesn't work.

Make a prominent note somewhere saying that this is the expected behavior, and that you can only sample offline mode when you're localhost or running a real HTTPS server.