add section on stop using route hooks for logic

[anotheredward]

This is the way we used to do things, but to ease transition to future versions of Ember, and because I've noticed others talk about it, we should stop this behaviour.

[anotheredward]

@samueljseay Cool, integrated your feedback :+1:

[anotheredward]

@JonForest Thanks for the comment, it's really useful to see that there's a reasonable consensus before adding stuff to the style guide!