Add default sort rules documentation

[kael89]

Thanks for this library, it's very useful! 💯

I believe it would be valuable to add documentation about the default rules used for sorting. For example, I was initially a bit confused by the behaviour described in #19. I only realized this was a feature and not a bug after I did some research in this repo and read the code.

I understand that this would essentially repeat a lot of information already in the code, but it may be valuable in the longer term, especially if the core logic does not change often. I'm happy to attempt a PR, if you are interested.

[keithamus]

PRs are very much welcome!