Add details on debug()ing Query and ResultSet objects.

Importing an excellent quick-reference of how to debug information in a Query/ResultSet from @lorenzo's [CakeFest 2015 talk](https://github.com/lorenzo/cakephp3-advanced-examples/blob/1dad4d52e44a7fadca47dc84c355ca31b431c828/slides.pdf) (slide 21). Copying [with permission](https://cakesf.slack.com/archives/general/p1433084148001526).