Add note to readme about comparator function

[mwgamble]

From what I can tell based on reading the source code, it appears that the comparator function must always be able to uniquely identify each node. By extension, it must be able to act as a stable sorting function. Is this accurate? If so, it would be great if a note regarding this could be added to the readme. I'd be happy to submit a PR to do this if you'd like.