The Arch Linux package installs to
/usr/share/zsh/plugins/zsh-syntax-highlighting/, not
/usr/share/zsh-syntax-highlighting/ as the generic "most Linux
distributions" instructions say -- verified against the package's
current file listing on archlinux.org. Added a dedicated Arch bullet
alongside the existing NetBSD/OpenBSD and Homebrew ones, in both
places the distro-specific paths are given.
Fixes#967
test-highlighting.zsh and test-perfs.zsh were linked as tests/test-*.zsh,
but this file already lives in tests/, so the links resolved to
tests/tests/test-*.zsh and 404'd. Found by pointing a markdown
link-checker at the repo.
