Add catalog best practice for keeping collections at consistent levels

[cholmes]

In discussion on radiantearth/stac-browser#46 - we think we don't need to revert the decision to not call everything 'catalog', but one thing that would help stac browser is if people keep their collection.json's on similar 'levels' of the hierarchy. It can then make better use of them. This is just a recommendation, so people can break it, but in general seems to be a nicer catalog if it is organized the same way consistently across it.