INSTALL.md: Mention more packages as dependencies for Arch

At least on Arch, none of the listed packages necessarily require that
the user has a working Xorg setup—this has already caused some confusion
for people.  In particular, xmessage is very much needed in order to
show warnings and compilation errors.

INSTALL.md

@@ -47,6 +47,7 @@ $ sudo dnf install \
 ``` console
 $ sudo pacman -S \
 > git \
+> xorg-server xorg-apps xorg-xinit xorg-xmessage \
 > libx11 libxft libxinerama libxrandr libxss \
 > pkgconf
 ```