Add packages for open-source headless CMS

https://github.com/netlify/netlify-cms

The `@saus/netlify-cms` package would export a `useNetlifyCms` function that hooks into Saus to add a renderer and a route (it uses `/admin/` by default). It might also inject state for pages with CMS data. I haven't looked much into how an integration would work, but perhaps it would also export helpers for creating "state fragments" (a new feature coming in `saus@0.5`) that load data from the CMS as well.

I would probably write the Saus docs with this. 🤔