docs: add the AUR install command to the README

downloader-bin 2.1.0-1 is now published on the AUR (first publish done —
the author registered his AUR account + this machine's SSH key, verified
the email, and the staged import was pushed). Future releases update it
automatically via release.sh submit_aur.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GpEAzmp4bQ5LDSW5ucuXk7

Author: bezzad

README.md

@@ -81,6 +81,11 @@ sudo snap install downloader
 curl -fsSL https://raw.githubusercontent.com/bezzad/Downloader.Desktop/main/scripts/install.sh | bash
 ```
 
+**Arch Linux** ([AUR](https://aur.archlinux.org/packages/downloader-bin)):
+```bash
+yay -S downloader-bin
+```
+
 **Windows** ([winget](https://learn.microsoft.com/windows/package-manager/winget/)):
 ```powershell
 winget install bezzad.Downloader