Add information about nginx

Few caveats with this commit:

1. I'm not sure if `.. code-block:: nginx` makes sense, but the preview pane here seemed to accept it (I'm making this commit via the github interface)

2. I said the client will always fallback to long-polling when it fails to make a connection via websockets. I'm not sure if it always will fallback to that, but in my testing (i.e. on *my machine*) that's what happened. For all I know it might decide to fallback to flash instead. I really don't know how the client side of socket.io works.