Update docker readme

https://github.com/apache/flagon/blob/master/docker/README.md

The read me includes a simple description and commands to run it. It would helpful to add more information to help people understand the example. This may be difficult for new contributors, but any additional information in the read-me would be appreciated.

1. The changes made from the [deviantony](https://github.com/deviantony/docker-elk) example for userale daga to ingest.
2. How to use the example.
1. Visit the example site to generate telemetry
2. Visit Kibana to view/query data