Added comment explaining how to start mongodb if it refuses to

@cosmoharrigan, in the future when attention allocation is used more often, I think it would be good if some modules that just require the use of a few methods of the ```attention_interface``` wouldn't need to start mongodb beforehand if they don't use it. At the moment, I added another comment explaining how to start mongodb. I think it would be good, though, if creating the mongodb connection would be put into its own method so a connection is instantiated only when required. What do you think, Cosmo?