docs: add notes about script globals

Author: makspll

docs/src/ScriptingReference/introduction.md

@@ -2,4 +2,11 @@
 
 This part of the book covers the user-facing API of the scripting languages supported by BMS. This will be where you will want to forward your script users to get started with scripting in BMS.
 
-If you are a modder, welcome! 👋, apologies for the rust-centricity of this guide, we are working on it!
+If you are a modder, welcome! 👋, apologies for the rust-centricity of this guide, we are working on it!
+
+## Globals
+
+Scripts will have access to a few global variables in most callbacks:
+- `world`: a static reference to the world, with all sorts of functions available
+- `entity`: the entity the script is attached to, not available on load/unload callbacks
+- `script_id`: the ID of the current script