Add deprecation notice to README.md

Author: vdavid

README.md

@@ -1,5 +1,16 @@
 # open-in-editor (Sourcegraph extension)
 
+## ⚠️ Deprecation notice
+
+**Sourcegraph extensions have been deprecated with the September 2022 Sourcegraph
+release. [Learn more](https://docs.sourcegraph.com/extensions/deprecation).**
+
+Starting with Sourcegraph 4.0, “Open in Editor” is a core part of the product. 🎉 Installing this extension is not needed and is not possible with 4.0+. For this reason, this extension is no longer in development.
+
+The repo and the old docs below are kept here to support older Sourcegraph versions.
+
+# Archive
+
 Adds a button to the Sourcegraph's extension panel and at the top of files in code hosts like GitHub (when the Sourcegraph browser extension is installed) that will open the current file in your editor of choice.
 
 ## Settings
@@ -191,4 +202,4 @@ Removes `sourcegraph-` from the final URL.
 1. Run `yarn && yarn run serve` and keep the Parcel bundler process running
 1. [Sideload the extension](https://docs.sourcegraph.com/extensions/authoring/local_development) (at the URL http://localhost:1234 by default) on your Sourcegraph instance or Sourcegraph.com
 
-When you edit a source file in your editor, Parcel will recompile the extension. Reload the Sourcegraph web page to use the updated extension.
+When you edit a source file in your editor, Parcel will recompile the extension. Reload the Sourcegraph web page to use the updated extension.