clarify steam install instructions

arch.md

@@ -398,6 +398,17 @@ sudo pacman -S qemu-guest-agent spice-vdagent
 
 <https://wiki.archlinux.org/title/Official_repositories#multilib>
 
+Edit /etc/pacman.conf
+
+```conf
+[multilib]
+Include = /etc/pacman.d/mirrorlist
+```
+
+```bash
+sudo pacman -S steam
+```
+
 When prompted, use vulkan-radeon
 
 ### XWayland