docs: REQUIREMENTS.md — OpenGL 4.3 floor, disk/RAM planning

Acer M5-481T fails Ghostty because Ivy Bridge HD 4000 tops out at
OpenGL 4.2 in Mesa; Ghostty needs 4.3 — a generation ceiling, not a
missing driver. Document that plus UEFI/x86_64, RAM, and storage floors
(Nix store generations + BTRFS snapper + hibernate swap≈RAM). Linked
from README, docs map, HARDWARE.md; #95 points here.

README.md

@@ -102,6 +102,10 @@ probably shouldn't exist.
 
 ## 2. Try it first (live ISO)
 
+**Before you install:** check **[docs/REQUIREMENTS.md](docs/REQUIREMENTS.md)**
+(UEFI x86_64, **OpenGL 4.3** for the default terminal, disk planning for the
+Nix store + BTRFS snapshots — 128 GiB is the comfortable floor).
+
 Boot the full desktop from a USB stick or VM without installing anything:
 
 ```sh