xss-lock: document an alternative using xsecurelock and mpv

Also, it would be neat if --panscan=1.0 was enabled in mpv
configuration.

bin/xss-lock

@@ -35,6 +35,10 @@ case "$1" in
         dunstctl set-paused true
         # Then, lock screen
         i3lock -n -e -i $HOME/.cache/i3/current-wallpaper.png -t -f
+        # Alternative if we prefer videos:
+        # XSECURELOCK_SAVER=saver_mpv
+        # XSECURELOCK_LIST_VIDEOS_COMMAND="ls -1 ~/.config/i3/wallpapers/*.mp4"
+        # xsecurelock
         echo "lock: unlock screen"
         # After unlocking screen, stop dimmer, restore notifications
         dimmer stop