Set a custom close message for archlinux/aur

It is experimental read-only mirror of the aur.git repository backing
the AUR, you aren't supposed to open PRs against it.

[1] infrastructure@68ec7871

close-messages.json

 {
+  "archlinux/aur": "This project is a experimental read-only mirror of the aur.git repository backing the AUR.",
   "archlinux/svntogit-packages": "This project is a read-only mirror.",
   "archlinux/svntogit-community": "This project is a read-only mirror.",
   "default": "This project is a read-only mirror of: ${upstream_url}, please submit your work there."