Add install instructions on Archlinux. (#1001)

---------

Co-authored-by: Carl Hörberg <carl@84codes.com>

Author: hugopl

README.md

@@ -45,7 +45,16 @@ EOF
 sudo dnf install lavinmq
 ```
 
-### macOS
+### Archlinux
+
+The package is available on [AUR](https://aur.archlinux.org/packages/lavinmq),
+it depends on `gc-large-config` (that conflicts with `gc` package package), it
+is the very same gc package found in ArchLinux but compiled with
+`--enable-large-config`.
+
+Then use systemctl to start/stop/enable/disable it, e.g. `systemctl start lavinmq`.
+
+### OS X
 
 Install LavinMQ with `brew`: