extend usage in readme

README.md

@@ -4,6 +4,7 @@ The original SSH executor can only be used for a single/static host described
 in the runner config. This is not as flexible as our use-case's needs.
 
 # Usage of the executor
+Edit your `.gitlab-ci.yml` to have a job something like this:
 ```yaml
 deploy:
   stage: your deploy stage
@@ -17,6 +18,8 @@ deploy:
     SSH_PORT: 10122                 # default: 22
 ```
 
+Add the ssh pubkey of the root@gitlab-runner-host (/root/.ssh/id_*.pub) to the remote machine (to some deploy user; /home/deployer-user/.ssh/authorized_keys).
+
 # Install the executor
 ```sh
 cd /opt