docs: add deprecation notice

Author: kdmccormick

README.md

@@ -1,3 +1,9 @@
+# ⚠️ DEPRECATION WARNING
+
+This repository is [deprecated](https://github.com/openedx/public-engineering/issues/276). It will no longer receive maintenance, support, or updates from the Open edX project.
+
+At the time of writing, edX.org still runs [a fork](https://github.com/edx/frontend-app-program-console) of this repository, but neither they nor the Open edX project make any guarantees about the stability or long-term availability of that code.
+
 # frontend-app-program-console
 
 # Purpose