docs(*): require platform domain setting; add warning to deisctl

```console
deisctl start platform
Note: The platform will fail to start if you haven't configured the domain used to serve applications.
For example: `deisctl config platform set domain=local3.deisapp.com`

● ▴ ■
■ ● ▴ Starting Deis...
▴ ■ ●

Data containers...
```

refs deis/deis.io#44