Added simple demo config

[akirill0v]

Added simple demo config and docker-compose example for starting experimentation.

Close #9.

✓ Checklist:

• This PR has been added to CHANGELOG.md (at the top of the list);
• Tests for the changes have been added (for bug fixes / features);
• Docs have been added / updated (for bug fixes / features).

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[mgrachev]

I think we should rename the demo/README.md file to CONTRIBUTING.md.

[akirill0v]

@mgrachev Thanks for your changes! I thought about your proposal and will make a text for contributing in future commits.