Add warning re. the need for disk encryption

This MR adds a warning to encourage people to encrypt their devices' local storage.

With the new security subsidiary rules, encryption of laptops/PCs/etc. is mandatory. We should encourage people to do this at install time, as re-installing after the fact is a major hassle. See also https://home.cern/news/news/computing/computer-security-dont-let-your-data-walk-away

Happy to adjust the wording.