add guidelines for printing stickers

[alicetragedy]

In the past, we used different services to order stickers. While we do have ready-to-go PDFs for the printers, it would be good to have at least a couple of finished templates that are service-specific, and to have related guidelines (which exact product to select, the amount we usually order, and so on) somewhere so that it's as easy as possible to order stickers.