Add Installation instructions for Archlinux

Pacman is the recommended installation method. And the package is in
the official repository, so makes sense to mention it in the README.

README.md

@@ -25,6 +25,13 @@ Coming soon!
 curl https://ollama.ai/install.sh | sh
 ```
 
+### Arch-Linux
+
+Ollama is available in the `extra` repository on Arch-Linux. This command is supposed to be run as root.
+```
+# pacman -S ollama
+```
+
 [Manual install instructions](https://github.com/jmorganca/ollama/blob/main/docs/linux.md)
 
 ### Docker