Update README.md to reflect extra steps for Etherpad configuration after skinning work

We shouldn't be calling these `contentObj` because they're just a shim. You can probably just make this:

```
var hostA = Etherpad.getPadUrl({'id': 'c:cam:abc123','etherpadPadId': 'padId'}, 'userId', 'sesionId', 'padId', 'authorId', 'language');
```