To doc: Add support for multiple composefile when deploying

[gbarr01]

References:
https://github.com/docker/docker-ce/blob/v18.03.0-ce-rc3/CHANGELOG.md
docker/cli#569

Possible doc location:
https://docs.docker.com/compose/extends/#understanding-multiple-compose-files
https://docs.docker.com/compose/production/

Closing this ticket due to its age, and the impending refactor. If you think this is in error, feel free to reopen. Thanks!

[docker-robott]

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