vars.md

Added a line documenting where to find acceptable values for the
`docker_version` setting. If you use a value that is not used as
a key value by `docker_versioned_pkg` the container-engine/docker
playbook will throw a "Unexpected templating type error". (e.g.
If you use '18.06.1' or '18.06.1-ce', neither of which is used
as a key value of `docker_versioned_pkg`, rather than '18.06',
you'll get an error when installing on Ubuntu 18.04.)