Suggest new devs review/update Hierarchy docs

This was my task from the Hydra Connect Core Developers breakout session:
https://groups.google.com/d/msg/hydra-tech/IAYTFjLp3ws/XIvDg6Y-tk0J
8. Write the "Encourage new developers to review Hierarchy and Documentation" in Contributing document of Hydra gem.

Is there anything more we want to say in this? Please feel free to suggest improvements.