Documentation: Add example of polymorphism/inheritance

[hrothwell]

Found that inheritance / polymorphism could be done, but not clear in README:

func save():
	save_dialog.show()
	
	save_dialog.file_selected.connect(func(path: String):
		var file = FileAccess.open(path, FileAccess.WRITE)
		file.store_line(ForgeJSONGD.class_to_json_string(my_abstract_resource_implementation, true)), # save the file type 
		ConnectFlags.CONNECT_ONE_SHOT)

func open():
	open_dialog.show()
	
	open_dialog.file_selected.connect(func(path: String):
		var previously_saved: AbstractResource = ForgeJSONGD.json_file_to_class(null, path) # null commented in scripts to use the saved type
		do_something(previously_saved), # this carries the previous implementation's type correctly
		ConnectFlags.CONNECT_ONE_SHOT)