docs: add deprecation notice

Author: taras-yemets

README.md

@@ -1,5 +1,14 @@
 # Sourcegraph code discussions
 
+## ⚠️ Deprecation notice
+
+**Sourcegraph extensions have been deprecated with the September 2022 Sourcegraph
+release. [Learn more](https://docs.sourcegraph.com/extensions/deprecation).**
+
+The repo and the docs below are kept to support older Sourcegraph versions.
+
+## Description
+
 Discuss code contextually, review code continuously.
 
 - 🖥️ Write comments on code while browsing on Sourcegraph.com.