Explain 4GB RAM requirement for setup computers

The setup computers will be running Ubuntu live, and need to download
the Ubuntu ISO to write to the Boot USBs. In a live system, the
writable filesystems are all RAM disks. The Ubuntu ISO is 1.6GB.

With only 2GB RAM, I got about halfway through the download before it
aborted with "No space left on device."

Fixes issue GlacierProtocol#25.

_docs/before-you-start/hardware.md

@@ -31,7 +31,7 @@ manufacturer than the drives for Set 1:
 
 ### Used/existing computing equipment
 
-* Two computers with Internet connectivity, administrator access, and about
+* Two computers with Internet connectivity, administrator access, at least 4GB RAM, and about
 2GB of free disk space.  **Each computer must be running Windows 10, macOS, or
 Linux.**