Add runtime configuration section to readme

[klingerf]

This branch updates the README with info about docker-specific environment variables that can be set to control a running docker container. I've also included links back to the Zipkin documentation of other available environment variables where appropriate. And I've added information about the minimum versions of docker / docker-compose required to run the v2 docker-compose files. I'm not super familiar with how all of the containers are configured, so please correct if I've made any mistakes.

[codefromthecrypt]

very good!

[klingerf]

Ok, made a few more wording tweaks to help to clarify some stuff in 1e1d20bf. After that's reviewed I think this should be good to go.

[codefromthecrypt]

looks great