About enabling multilib on arch based distros (#593)

* Update README.md about Arch Linux multilib

README.md

@@ -57,6 +57,24 @@ If you do not wish to compile anything, simply download the file under [Releases
 
 If you are using an Arch-based distribution, install [`mangohud`](https://aur.archlinux.org/packages/mangohud/) and [`lib32-mangohud`](https://aur.archlinux.org/packages/lib32-mangohud/) with your favourite AUR helper. [`mangohud-git`](https://aur.archlinux.org/packages/mangohud-git/) and [`lib32-mangohud-git`](https://aur.archlinux.org/packages/lib32-mangohud-git/) are also available on the AUR if you want the up-to-date version of MangoHud.
 
+If you are building it by yourself, you need to enable multilib repository, by editing pacman config:
+
+```
+sudo nano /etc/pacman.conf
+```
+
+and uncomment:
+
+```
+#[multilib]
+#Include = /etc/pacman.d/mirrorlist
+```
+
+then save the file and execute:
+```
+sudo pacman -Syy
+```
+
 #### Debian 11 (Bullseye)
 
 If you are using Debian 11 or later, to install the [MangoHud](https://tracker.debian.org/pkg/mangohud) package, execute: