docs: document elasticsearch.yml location for xpack.security

[vee1e]

Fixes #34

This adds the missing answer for where to put elasticsearch.yml when you want to turn on security. By default PcapMonkey does not ship that file. The search service runs from a Docker image with its config inside the container.

What changed:

• README now shows how to create config/elasticsearch/elasticsearch.yml, with the three defaults the image expects plus xpack.security.enabled. Mounting a file with only the last line hides the defaults and the service may not start.
• Notes that the file must exist before you start the stack, otherwise Docker makes a folder with that name.
• Shows the mount in the right place under the search service, keeping the data folder.
• Restart now covers the whole stack, and warns that turning on security without a password breaks the log shippers that talk to the search service.
• Includes the no-file alternative using an environment setting, and notes the ARM image tag differs.

[CLAassistant]

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