docs: Update 0.9.9.md

Author: makspll

release-notes/0.9.9.md

@@ -4,7 +4,24 @@
 
 ## Summary 
 - Dynamic Systems are here! [^1]
-  - TODO
+    - it's now possible to query schedules, systems, and inject new event handlers from scripts themselves:
+  ```lua
+    local post_update_schedule = world.get_schedule_by_name("PostUpdate")
+
+    local test_system = post_update_schedule:get_system_by_name("on_test_post_update")
+
+    local system_after = world.add_system(
+        post_update_schedule,
+        system_builder("custom_system_after", script_id)
+            :after(test_system)
+    )
+
+    local system_before = world.add_system(
+        post_update_schedule,
+        system_builder("custom_system_before", script_id)
+            :before(test_system)
+    )
+  ```
 - `WithWorldGuard` and `HandlerContext<Plugin>` system parameters can be used in conjunction to create custom event handler systems
    ```rust
    fn my_handler(with_guard: WithWorldGuard<HandlerContext<Plugin>>) {