Add notes explaining limitations and updates.

@doctorpangloss I don't think "cannot use more than one dyno" is correct. The issue you're referring to is that clients who aren't using websockets fall back to http requests (see [here]https://github.com/meteor/meteor/blob/master/packages/livedata/stream_client_sockjs.js#L132), and those may go to a different dyno. 

Correct me if I'm wrong, but I believe the only issue with that is that DDP updates may be delayed if they're based on Mongo poll-and-diff. And setting up oplog tailing should take care of most of these situations.

Websockets connections *are* sticky.

So I think the only issue is that clients that don't support websockets may have delays on only some DDP updates if oplog tailing is not enabled.