add env-vars documentation

[jasonzubiate]

This change resolves #31 by adding documentation for env-vars. Signed-off-by: Jason Zubiate jzubiate.dev@gmail.com

[workingjubilee]

This will somewhat duplicate the effort of, and has conceptual conflicts, respectively, with

• https://github.com/Gankra/cargo-mommy/pull/41
• https://github.com/Gankra/cargo-mommy/pull/43