yaourt -S mkinitcpio-overlayfs
To enable the hook you need to edit the configuration file /etc/mkinitcpio.conf. With this you can customize your ramdisk with additional modules, scripts, files and what not.
However, after a clean Arch Linux installation you will find a line which looks like this:
HOOKS=(base udev autodetect modconf block filesystems keyboard fsck)
Just put overlayfs at the right time / position you want to run it, like below. In this example it will be executed last.
HOOKS=(base udev autodetect modconf block filesystems keyboard fsck overlayfs)
Finally run the following command to rebuild your ramdisk.
mkinitcpio -P