Addendum

I've added the HTML formatted version of the Addendum as authored by @JWDebelius. This is likely not ready to be merged. I've made it available via the [following page](http://webdev.microbio.me/americangut/addendum.psp), to get to this page users will have to login with their account and click in the left sidebar "The American Gut Addendum", which should open a new page with all the information that's currently contained in this document.

@JWDebelius, would you mind taking a look at it and making sure everything is looking good?