Clarify the relationship between the Pug Website repo and the Pug documentation repo.

[audreytoskin]

At first glance, it seems like the repositories pug-www, pug-en, and pug-zh-cn might all be the same thing. I mean, I think I was able to figure it out: From looking through the pug-www and pug-en repositories, I gather that the Pug documentation repos contains most of the actual content (the language and API spec), and the Pug website repo adds some extra templates and scripts for presenting the documentation online. Still, it might help to state in their respective READMEs what role each repository serves. Users who see a typo in the documentation, or a bug in the interactive code examples, might rush to the wrong repository and get confused.