Readme out of date...?

Following the instructions in the readme:

`mix release --env=prod`
gives:
`mix release --env=prod
** (Mix) Could not invoke task "release": 1 error found!
--env : Unknown option`

`mix help release'
gives:
`## Command line options

• --force - forces recompilation
• --no-archives-check - does not check archive
• --no-deps-check - does not check dependencies
• --no-elixir-version-check - does not check Elixir version
• --no-compile - does not compile before assembling the release
• --overwrite - if there is an existing release version, overwrite it
• --path - the path of the release
• --quiet - do not write progress to the standard output
• --version - the version of the release`

Following the rest of the readme I can't produced the tar file so I'm kind of lost.