Document how to start ES with less RAM

[stchris]

Suggestion from Slack:

The answer to my question about getting ElasticSearch to start in a docker instance (which disappears immediately when there's an error so you can't connect to it via shell) to use less memory is to add the following to docker_compose.yml in the elasticsearch: -> environment: section:

- ES_JAVA_OPTS=-Xmx4g -Xms4g

Where "4g" represents 4GB of RAM as opposed to the default 16GB.
It would be helpful if the documentation mentioned something about this since not every server may have enough RAM to support that significant default.