update usage guide

USAGE_GUIDE.md

@@ -2,6 +2,14 @@
 
 This guide walks you through building the Auracaster system, flashing it to an SD card, and performing an A/B update.
 
+## Prerequisites
+
+Before building, install the required packages (for Ubuntu/Debian):
+
+```bash
+sudo apt update && sudo apt install -y build-essential git rsync bc python3 unzip file wget cpio libssl-dev libncurses5-dev flex bison libelf-dev device-tree-compiler u-boot-tools
+```
+
 ## Building the System
 
 1. Clone or navigate to your Buildroot repository: