The document recommends a non-durable location for data

I noticed the documentation recommends a non-durable local resource for the Elasticsearch data path.  Although this is acceptable for some deployments it might be worth warning people that the path is not durable and there is a potential for data loss, even with replicas data loss is theoretically possible.

```
# recommended
path.data: /mnt/resource/elasticsearch/data
```

Alternatively the user could attach and use data disks which do come with a significant performance tradeoff, but premium storage options with higher IOPS have been announced and are right around the corner.

Closes #46.

(cherry picked from commit 27a1206)