Mention the Arch Linux PKGBUILD in README.rst

This Pull Request is entirely optional and cosmetic. I made a PKGBUILD for Arch Linux to simplify the installation of RPIO. Using this file, Arch users only need to do

    wget https://aur.archlinux.org/packages/rp/rpio/PKGBUILD
    makepkg -i

to install RPIO on their system or 

    pacman -R rpio

to cleanly remove it again.