docs: add deprecation notice

Author: taras-yemets

README.md

@@ -4,6 +4,17 @@
 [![codecov](https://codecov.io/gh/sourcegraph/sourcegraph-git-extras/branch/master/graph/badge.svg?token=c3KpMf1MaY)](https://codecov.io/gh/sourcegraph/sourcegraph-git-extras)
 [![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 
+## ⚠️ Deprecation notice
+
+**Sourcegraph extensions have been deprecated with the September 2022 Sourcegraph
+release. [Learn more](https://docs.sourcegraph.com/extensions/deprecation).**
+
+Starting with the September 2022 release of Sourcegraph, git extras is a core part of the product.
+
+The repo and the docs below are kept to support older Sourcegraph versions.
+
+## Description
+
 A [Sourcegraph extension](https://docs.sourcegraph.com/extensions) that adds useful features when viewing files in a Git repository on [Sourcegraph](https://sourcegraph.com), GitHub, GitLab, and other [supported code hosts](https://docs.sourcegraph.com/extensions):
 
 - **Git: Show/hide blame**: toggles Git blame annotations with each line's last commit, author, date, etc.