Add Arch dependencies to the README

Add instructions for installing dependencies on Arch Linux and
derivatives.

On Arch, the texlive-most group depends on most TeX Live related
packages, texlive-core appears to be the bare minimum install for
ConTeXt.

See Arch Wiki on TeX Live: https://wiki.archlinux.org/index.php/TeX_Live

README.md

@@ -31,3 +31,8 @@ sudo apt install pandoc context
 ```bash
 sudo dnf install pandoc texlive-collection-context
 ```
+
+#### Arch
+```bash
+sudo pacman -S pandoc texlive-core
+```