Default styling paragraph (preserve intrinsic aspect ratios)

### Comments

The added paragraph is a straightforward adaptation of the corresponding paragraph in the `<video>` spec.

### Questions

I almost changed some bits in the long note on the "algorithm for deriving the source image", too. Namely:

- the bit about `<picture>` behaving "exactly the same" as `<img>`, with the "only difference" being the multiple-sources & picking mechanics. This aspect-ratio thing might be a significant additional difference?
- adding `background-size: contain; background-position: center;` to the "rough CSS equivalent"

But I figured I'd touch as little as possible. Thoughts?

More mundanely, I feel a little bit technically in over my head here...

Should I have run the gen.js script? I managed to figure out that it was a node thing and install and run node, but the script failed because I didn't have a thing called "anolis", which some googling told me I may have to install mercurial, python, lxml, cssselect, html5lib, and a package manager to get, which, I mean, woah.

Also when I pushed from my local repo to my github repo I got a friendly email from an octocat telling me that a page build had failed because a CNAME (picture.responsiveimages.org) was already taken, which, okay, what *is* that little CNAME file doing in there anyways?

Apologies for being new to this & needing a little hand-holding...