Order nested association documentation

PR for putting documentation of #1250 onto the sequelize site so you don't have to crawl google to find the information.

Really it isn't much, mostly just copy pasting @overlookmotel 's first post into the doc, but it should be good enough to help those who want to use this.. To use this.

And apologies for the double post sort of thing, as my IDE decided to shove one of its files into the git before.