Add warning to storybook README

packages/storybook/README.md

@@ -2,7 +2,11 @@
 
 Private package containing the Storybook for the Nimble repo.
 
+## Usage warning
+
+This package is the exception to normal package dependency rules by having explicit monorepo path dependencies on nimble-components and spright-components and by inspecting non-public contents such as typescript source code instead of public build artifacts. The package should be not be used as a model of best practices for the nimble monorepo.
+
 ## Contributing
 
 See `Getting Started` in [`CONTRIBUTING.md`](/packages/storybook/CONTRIBUTING.md) to get started with building
-and running storybook.
+and running storybook.