Add FAQ about old versions being unsafe

Often at Helpdesk we get users with hardware problems asking where can they download previous versions of Tails, because the current version is not working on their hardware.

Maybe we could have an FAQ saying something along the lines:

Where can I find old versions of Tails

Please don't install an old Tails version, as they contain vulnerabilities that are well known, and are unsafe to use. To be safe you need to run the last version of Tails.

or something like that.