Add Docker quickstart or example reference to readme

[dkisselev]

The Readme includes the run steps for starting the exporter directly but the Docker containers behave a little differently which lead to some confusion before I found the compose examples nested in the project.

Specifically, the entrypoint is the start.sh script, which looks for environment variables to declare the single device settings or config file.

As a user, I expected to be able to run something like docker run mikrotik-exporter "--config-file <config-file" using the same argument structure that was in the readme.

I would recommend adding a "Running with Docker" section that lists the available/required environment variables.