Exposing right ssh port & short info on ssh usage

Follow up for #7, which changed sshd port from default 22 to 2122

I wish I had more to write about ssh usage, but for me it's not quite clear how one is supposed to log-in. I am using quite hacky way, copying generated ssh keys from container: 

```
docker cp $CONTAINER_ID:/root/.ssh/id_rsa ~/.ssh/hadoop-docker/
ssh -i ~/.ssh/hadoop-docker/id_rsa root@$DOCKER_HOST -p 2122
```

If I am doing this wrong way, please correct me, and I will update the doc.