doc enhancement/update : ember-addon section must be put in ember section

Hello,

ember-addon section must be put in ember section from package.json :

```
"ember": {
    "edition": "octane",
    "ember-addon": {
      "configPath": "wayto/config",
      "rootURL": "https://user.github.io/repository-name"
    }
  }
```

Is it the good way ? 

Does the doc need update ?

Thanks,

Eric
