Add spawn-at-startup config option

Fixes https://github.com/YaLTeR/niri/issues/12

resources/default-config.kdl

@@ -35,6 +35,11 @@ input {
     scale 2.0
 }
 
+// Add lines like this to spawn processes at startup.
+// Note that running niri as a session supports xdg-desktop-autostart,
+// which may be more convenient to use.
+// spawn-at-startup "alacritty" "-e" "fish"
+
 binds {
     // Keys consist of modifiers separated by + signs, followed by an XKB key name
     // in the end. To find an XKB name for a particular key, you may use a program