Add a README to link to the GDScript integration tests documentation

This makes the documentation about creating and running GDScript
integration tests more discoverable.

modules/gdscript/tests/README.md

@@ -0,0 +1,8 @@
+# GDScript integration tests
+
+The `scripts/` folder contains integration tests in the form of GDScript files
+and output files.
+
+See the
+[Integration tests for GDScript documentation](https://docs.godotengine.org/en/latest/development/cpp/unit_testing.html#integration-tests-for-gdscript)
+for information about creating and running GDScript integration tests.