Update index.qmd

Add under-construction warning

Author: andypbarrett

index.qmd

@@ -5,4 +5,12 @@ author: NSIDC
 
 ## Welcome
 
-Welcome to the NSIDC Data Cookbook!
+Welcome to the NSIDC Data Cookbook!
+
+<!-- prettier-ignore -->
+> [!WARNING]
+> This cookbook is under active development. Major changes to the structure of
+> the book and its content are expected. We are striving to develop content
+> that is well-tested and peer-reviewed, but nothing contained here should be
+> expected to work correctly (or at all!) in this early phase.  Many sections contain only an outline of the content.
+> These sections will have content added at the project develops.