Add warning for non-production use in README

Added a warning about the component's intended use.

Author: ritazh

tools/metal/storage/README.md

@@ -1,3 +1,5 @@
 A very simple s3 mock to support our testing of bare metal / VMs.
 
-Just enough functionality to support our tests.
+Just enough functionality to support our tests.
+
+WARNING: This component is not intended for real-world deployment. DO NOT USE for production.