mix espec.init doesn't work in umbrella apps

Hi!
I'm trying to start using espec in umbrella application. Because of there are no information in README, i've just done the following steps

1 - added espec to the deps section in main app, added preferred_cli_env
2 - added espec to the every mix.esx in apps folder

when I use `mix espec.init` in any app folder i see following message

```
** (Mix) The task "espec.init" could not be found
```

What's wrong? May I ask you to add in README file information how to use espec in umbrella apps?