Update Web NFC Not Available to include HTTPS warning

[MisterTeknis]

On Android Chrome (have not tested others) I was only able to use the Web NFC functionality in SpoolmanSync when accessed over a HTTPS connection. It might be worth adding that in the "Web NFC Not Available" message for self hosters that run into similar issues.

[gibz104]

Thanks for letting me know! I've added a note about that in the README.md.