docs: add deprecation notice

Author: taras-yemets

README.md

@@ -7,6 +7,15 @@
 [![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 [![semantic-release](https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg)](https://github.com/semantic-release/semantic-release)
 
+## ⚠️ 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
+
 Stubs for the Sourcegraph extension API to unit-test Sourcegraph extensions
 
 ## Install