All future model-specific installation guides can be stored in
`installing/models/`. Once there is at least one complete guide, we can
start linking to the model-specific installation guides (whether by
adding a new section to the existing `installation-guide.md` or by
creating a new page listing all of them).
This idea was proposed by user Piit (piitberlin), who has volunteered to
create a guide for the Lenovo W540. Thanks, Piit!
https://groups.google.com/d/msg/qubes-users/ej11850J4UI/AXq8ufI2CwAJ
