update arch linux installation instructions (now available via pacman)
update arch linux installation instructions (now available via pacman)

closes #328

joshuarubin/dotfiles,zsh-users/zsh-autosuggestions,ascenator/zsh-autosuggestions,joshuarubin/dotfiles,tarruda/zsh-autosuggestions,joshuarubin/dotfiles,zsh-users/zsh-autosuggestions,joshuarubin/dotfiles