add docs about callbacks

Author: makspll

docs/src/SUMMARY.md

@@ -17,3 +17,4 @@
     - [ScriptTypeRegistration](./ScriptingReference/script-type-registration.md)
     - [ScriptQueryBuilder](./ScriptingReference/script-query-builder.md)
     - [ScriptQueryResult](./ScriptingReference/script-query-result.md)
+- [Core Callbacks](./ScriptingReference/core-callbacks.md)

docs/src/ScriptingReference/core-callbacks.md

@@ -0,0 +1,28 @@
+# Core Callbacks
+
+On top of callbacks which are registered by your application, BMS provides a set of core callbacks which are always available.
+
+The two core callbacks are:
+- `on_script_loaded`
+- `on_script_unloaded`
+
+## `on_script_loaded`
+
+This will be called right after a script has been loaded or reloaded. This is a good place to initialize your script. You should avoid placing a lot of logic into the global body of your script, and instead put it into this callback. Otherwise errors in the initialization will fail the loading of the script.
+
+```lua
+print("you can also use this space, but it's not recommended")
+function on_script_loaded()
+    print("Hello world")
+end
+```
+
+## `on_script_unloaded`
+
+This will be called right before a script is unloaded. This is a good place to clean up any resources that your script has allocated. Note this is not called when a script is reloaded, only when it is being removed from the system.
+
+```lua
+function on_script_unloaded()
+    print("Goodbye world")
+end
+```