add instructions to checkout the correct version of airbyte (#1051)

docs/getting-started-tutorial.md

@@ -15,6 +15,8 @@ First of all, make sure you have Docker and Docker Compose installed. Then run t
 ```text
 git clone https://github.com/airbytehq/airbyte.git
 cd airbyte
+git fetch --tags
+git checkout tags/v0.5.1-alpha
 docker-compose up
 ```