Add note to docs re: early pre-alpha

tutorial/index.rst

@@ -6,6 +6,13 @@
 JupyterLab Tutorial
 ===================
 
+.. important::
+    
+    This is a very early pre-alpha developer preview and not suitable for
+    general usage yet. Features and implementation are subject to change.
+
+
+
 Contents:
 
 .. toctree::