Add a section to the README documenting how environment variables are used by dbmate

As a consequence of #491, it would be helpful to add a section to the README that describes and explains how dbmate uses environment variables.

* Which settings are derived from which environment variables
* How dbmate loads from `.env` in the current working directory by default
* How to specify one or more dot-env files using the `--env-file` option
* Variable precedence: dot-env files, the calling process's environment, command line option value