add yay install instructions to base

arch/base.md

@@ -14,6 +14,7 @@ I have instructions for building a:
   - [Installation](#installation)
     - [Preparation](#preparation)
     - [Boot](#boot)
+    - [AUR](#aur)
     - [Security](#security)
       - [Secure Boot](#secure-boot)
       - [TPM2 LUKS Decryption](#tpm2-luks-decryption)
@@ -229,6 +230,17 @@ Follow most of the instructions here:
     Exec = /usr/bin/systemctl restart systemd-boot-update.service
     ```
 
+### AUR
+
+If you ever need anything from the AUR, yay is the tool to install it.
+
+```bash
+pacman -S --needed git base-devel
+git clone https://aur.archlinux.org/yay.git
+cd yay
+makepkg -si
+```
+
 ### Security
 
 <https://wiki.archlinux.org/title/security>