README suggestion to ensure DISPLAY is set, is redundant with distribution best practices

[eli-schwartz]

Based on discussion at gentoo/gentoo#38031 (comment), with reference to the advice offered in 8538e01

Distros usually have a stock xinitrc sample which imports DISPLAY into the user environment for you. Users who write their own xinitrc or or use alternative approaches to starting the X server and running a window manager should be encouraged to load the distro-wide hook functions (often by copying the default xinitrc and customizing it rather than starting from scratch).

Perhaps say instead, "for example your xinitrc should probably load the distro hooks at /etc/X11/xinit/xinitrc.d/*, one of which most likely runs ${systemctl_command}"

[cdown]

(I'm in Uganda for a while with mobile only, remind me later if I don't get back :-))