man: mention logind for launching

Logind has been long supported as means to launch Weston without
weston-launch. It's good to note it in the manual.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

man/weston.man

@@ -20,7 +20,10 @@ shell.
 When weston is started as the first windowing system (i.e. not under X nor
 under another Wayland server), it should be done with the command
 .B weston-launch
-to set up proper privileged access to devices.
+to set up proper privileged access to devices. If your system supports
+the logind D-Bus API and the support has been built into weston as well,
+it is possible to start weston with just
+.BR weston .
 
 Weston also supports X clients via
 .BR XWayland ", see below."