updated README

I noticed a few things, so I fixed them... 
I couldn't get it to work with 
```js
Presences.find({ currentRoomId: Session.get('currentRoomId') });
```
not sure if I was doing something wrong, but I've changed documentation to what I got it working with.

```js
Presences.find({ state: { online: true, currentRoomId: Session.get('currentRoomId') } });
```