Add deprecation notice

Author: vdavid

README.md

@@ -1,5 +1,14 @@
 # [SonarQube](https://www.sonarqube.org/) Sourcegraph extension
 
+## ⚠️ 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
+
 Show SonarQube issues when browsing files on Sourcegraph.
 
 <p>