Specifically declare that :tags aren't nested inside of the :articles resource

My code was breaking because I had put the :tags resource inside of the :articles resource block.

Doing this caused the error `undefined_method tag_path`.

I changed the docs to specifically state that the :tags resource should *not* be nested inside of the :article resource.
Merge pull request #727 from LouisaBarrett/patch-1

Specifically declare that :tags aren't nested inside of the :articles re...