Update README.md: add deprecation warning. (#27)

Author: samdutton

README.md

@@ -1,3 +1,14 @@
+**Some Privacy Sandbox technologies are being phased out**.
+
+See our [Update on Plans for Privacy Sandbox Technologies](https://privacysandbox.com/news/update-on-plans-for-privacy-sandbox-technologies/).
+
+[Privacy Sandbox feature status](https://privacysandbox.google.com/overview/status) provides more information about the status of individual APIs and
+platform features.
+
+This repo is scheduled to be archived.
+
+---
+
 # Project Flight: README
 
 Project Flight is a sample project that combines the privacy-preserving APIs of