Add a package update command (#2205)

* Add a package update command

when running these commands on archlinux-base proxmox lxc, found out that these commands return 404 error unless the pacman -Sy command is run.

This although seeming simplistic might be something unknown when someone uses yay and archlinux for the first time.

* Update README.md

README.md

@@ -35,10 +35,10 @@ If you are migrating from another AUR helper, you can simply install Yay with th
 The initial installation of Yay can be done by cloning the PKGBUILD and
 building with makepkg:
 
-Before you begin, make sure you have the `base-devel` package group installed.
+We start with updating the package lists and make sure we have the `base-devel` package group installed.
 
 ```sh
-pacman -S --needed git base-devel
+pacman -Sy --needed git base-devel
 git clone https://aur.archlinux.org/yay.git
 cd yay
 makepkg -si