readme: add arch aur package to readme

[pjvds]

I'm maintaining the Arch package for base16-builder-go and this pull requests adds an install entry to the README.

[belak]

Hi, thanks for submitting this! I'm not sure what the best route forward with this is - I tend to shy away from recommending aur helpers in readmes because they tend to change every few years and they all have different trade-offs (I've been using auracle + makepkg for a while, but that's not as full-featured as others).

However, I do appreciate you putting together a PKGBUILD and taking the time to do this. Maybe just linking to it and telling them to install it from the AUR, or maybe even with their favorite AUR helper would be a good middle ground?

[JamyGolden]

I think this makes sense to add some kind of installation instructions to the readme. I think an also including a “## Manual” installation instruction mentioning copying the binary to some $PATH directory would be good too.