add post_startup stage

crates/bevy_app/src/app_builder.rs

@@ -164,6 +164,7 @@ impl AppBuilder {
 
     pub fn add_default_stages(&mut self) -> &mut Self {
         self.add_startup_stage(stage::STARTUP)
+            .add_startup_stage(stage::POST_STARTUP)
             .add_stage(stage::FIRST)
             .add_stage(stage::EVENT_UPDATE)
             .add_stage(stage::PRE_UPDATE)

crates/bevy_app/src/stage.rs

@@ -1,6 +1,9 @@
 /// Name of app stage that runs once when an app starts up
 pub const STARTUP: &str = "startup";
 
+/// Name of app stage that runs once after startup
+pub const POST_STARTUP: &str = "post_startup";
+
 /// Name of app stage that runs before all other app stages
 pub const FIRST: &str = "first";