Add 'GDscript Autoloads'

GDscript-Autoloads.md

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+```gd
+onready var util = get_node("/root/Util")
+```
+File util.gd
+
+Contains common helper functions that can be used in multiple places in the codebase
+
+```gd
+onready var global_event_bus = get_node("/root/GlobalEventBus")
+```
+File: global_event_bus
+Usage Example: effects/SinkBlock.gd
+
+A place to put signals that need to be sent to between objects that don't share a scene