Convert Trail3D to script class. Update README.

README.md

@@ -74,5 +74,6 @@ That's it! I hope you've got ideas of what you'd like to share with others.
 |[ResourceSet](addons/godot-next/resources/resource_collections/resource_set.gd)|A ResourceCollection implementation that manages a Set of Resources.|GDScript
 |[Singletons](addons/godot-next/global/singletons.gd)|A utility for caching Reference-derived singletons. Resources with a `SELF_RESOURCE` constant with a path to a `*.tres` file will be automatically loaded when accessed.|GDScript
 |[Trail2D](addons/godot-next/2d/trail_2d.gd)|Creates a variable-length trail that tracks a "target" node.|GDScript
+|[Trail3D](addons/godot-next/3d/trail_3d.gd)|Creates a variable-length trail on an ImmediateGeometry node.|GDScript
 |[Variant](addons/godot-next/global/variant.gd)|A utility class for handling Variants (the type wrapper for all variables in Godot's scripting API).|GDScript
 |[VBoxItemList](addons/godot-next/gui/v_box_item_list.gd)|Creates a vertical list of items that can be added or removed. Items are a user-specified Script or Scene Control.|GDScript

addons/godot-next/3d/trail_3d.gd

@@ -7,6 +7,7 @@
 # - shape: curve used to shape trail, right click on this in inspector to see curve options
 
 extends ImmediateGeometry
+class_name Trail3D
 
 export(float) var length = 10.0
 export var max_radius = 0.5