Updated Readme [skip ci]

README.md

 
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+## Installation
+
+You can get some not-so-recent Mac releases [on the GitHub page](https://github.com/xythobuz/OpenRaider/releases).
+
+The most recent OpenRaider version can be found on the [Arch Linux User Repos](https://aur.archlinux.org/packages/openraider-git/). Install it with your favorite aur-helper like this: `yaourt -S openraider-git`.
+
 ## Configuration
 
 OpenRaider needs some configuration files, and level data and assets from custom levels or the Tomb Raider games.