docs: Add disclaimer to README.md

Author: drstrangelooker

README.md

@@ -1,4 +1,9 @@
-# Looker CLI (looker-cli) Go Implementation
+**This is not an officially supported Google product. This is supported at a
+Best Effort level. Use at your own risk.**
+
+---
+
+# Looker CLI (looker-cli)
 
 Looker CLI (`looker-cli`) is a robust, fast command-line interface (CLI) tool designed to navigate, manage, and automate Looker resources (Folders/Spaces, Looks, Dashboards, Users, and more) via the Looker API 4.0.