Documentation - Different environment variables per stage

Hello,

It seems to have been done in the #353 but I don't see it in the documentation and I tried several things but not successful.

What I tried :

```
"stages": {
"staging": {
"ENDPOINT": "http://dev.com",
"environment": {
"ENDPOINT": "http://dev.com"
}
}
```

Do someone have a clue ? :-)
Maybe it's only implemented for UP PRO ?