add very basic readme (#48785)

* add very basic readme

apps/editing-toolkit/editing-toolkit-plugin/wpcom-block-editor-welcome-tour/README.md

@@ -0,0 +1,7 @@
+# Nux Welcome Tour Modal
+
+A help tour to show new users some of the basics of using the editor.
+
+### Testing Instructions
+
+Instructions for testing the modal, and for resetting the state of `nux-status` so that the modal is shown again can be found here https://github.com/Automattic/wp-calypso/pull/47779