Outline goals and/or non-goals

`document.createElement('foo')` doesn't exist in a vacuum. Outline what is, and is not, in scope. This would clarify what's meant by _minimally viable_ and save a lot of back-and-forth in issues/PRs.

If a list of criteria/props isn't desired, a paragraph or two on why this was created the purpose it serves would be educational.

e.g. should this work with `d3`, `jQuery`, `cheerio`, etc? Which DOM props/attrs are supported and why? Same for unsupported props/attrs (e.g. `style`, `on*`, etc).