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

[dossy]

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