Documentation: include mix coveralls.html preferred_cli_env

[nathany]

I found it a bit confusing after copy/pasting some config and running:

mix coveralls.html

to get this error:

** (Mix) The task "coveralls.html" could not be found

why not include it in the README:

preferred_cli_env: ["coveralls": :test, "coveralls.detail": :test, "coveralls.html": :test, "coveralls.post": :test],

[parroty]

Thanks for the comment. I forgot to update when "coveralls.html" was added later.

[nathany]

Thanks