Document that the action requires to run on ubuntu

[jorisvandenbossche]

I naively added the action to our cibuildwheel matrix (for the shapely wheels), expecting to upload the wheels from each OS-specific build directly.
However, this does not work because the action apparently requires a linux image: "Container action is only supported on Linux" (see eg https://github.com/shapely/shapely/actions/runs/8047467718/job/21976750565). So you need to gather all wheel for the different architectures and upload them from another job running on ubuntu.

[matthewfeickert]

Ah yes. I hadn't thought about that, but as it is container based, it currently requires a Linux OS base.

This should get documented in the README as you suggest.

[matthewfeickert]

Note to self: c.f. shapely/shapely#1998