To install another OS, follow the instructions at https://www.chromium.org/chromium-os/developer-information-for-chrome-os-devices/chromebook-pixel-2015 to enable developer mode, after which you should be able to hit Ctrl+L on the boot screen to launch SeaBIOS, which will boot external media (BIOS only, not GPT). You’ll be able to boot the Arch installation image from a USB stick using this.
Currently, I’ve got GRUB running off an SD card, which I boot using SeaBIOS. My system is installed on the internal drive, but GRUB won’t work from there. My technique at https://gist.github.com/bodil/b14a398189e5643ee03e has so far failed to yield a booting kernel on the internal drive, which is somewhat annoying, but as I keep the SD card permanently inserted anyway, it hasn’t been bothersome.
Arch has a wiki page on the Pixel 2015: https://wiki.archlinux.org/index.php/Chromebook_Pixel_2
The stock kernel (as of 4.0.5) lacks support for some h