Document how to create your own services targeting a subset of the ES nodes

[sebgl]

Common use cases:

• a dedicated service for ingest nodes (only)
• a service that does not include master nodes
• a service that spans over multiple nodeSets

It's pretty easy for the user to create their own services according to their needs, and pretty hard for us to come up with additional default services on top of the one we create targeting all pods.

Let's document how to do it.