Packaging note for Arch Linux

[randrej]

You might want to note somewhere in the readme that the library is packaged for Arch Linux in the AUR (by me).

Another thing: I've packaged the git version (master), and not the tags, because the project isn't released frequently. It'd be nice if you released a new official version, so that I can make a proper versioned package.

[variar]

Repology provides nice badge with overview of available packages, and it updates automatically:

Markdown for it:

[![Packaging status](https://repology.org/badge/vertical-allrepos/namedtype.svg)](https://repology.org/project/namedtype/versions)