Migrate Google-Docs-based documetation into a Markdown-based README.md.

Migrate Google-Docs-based documetation into a Markdown-based README.md. Auto-TOC generated with http://doctoc.herokuapp.com/ - changes to the structure of the markdown will need the TOC to be manually fixed up or re-generated.

Github's gh-pages can use a special syntax

```
* TOC placeholder (To be scraped)
{:toc}
```

... but the basic .md interpreter for in-tree code doesn't use the markeru system that has these extensions, so the TOC is manual for now.