Added compatibility mode section in docs

Author: ezimuel

docs/connecting.asciidoc

@@ -201,6 +201,21 @@ $client = ClientBuilder::create()
 ----
 
 
+[discrete]
+[[client-comp]]
+=== Enabling the Compatibility Mode
+
+The Elasticsearch server version 8.0 is introducing a new compatibility mode that
+allows you a smoother upgrade experience from 7 to 8. In a nutshell, you can use
+the latest 7.x Elasticsearch client with an 8.x Elasticsearch server, giving more
+room to coordinate the upgrade of your codebase to the next major version.
+
+If you want to leverage this functionality, please make sure that you are using the
+latest 7.x client and set the environment variable `ELASTIC_CLIENT_APIVERSIONING`
+to `true`. The client is handling the rest internally. For every 8.0 and beyond
+client, you're all set! The compatibility mode is enabled by default.
+
+
 [discrete]
 [[client-usage]]
 === Usage