Set up Flatdoc to render README.md as a GitHub page

You could just leave this index.html can be in its own `gh-pages` branch.

I would rename the `master` branch to `gh-pages` and under Settings make `gh-pages` the default branch.

Related to grant/swift-cheat-sheet#15.

Also:

- Instead of using `---` for `<hr>`, use `***`. This prevents a bug in Flatdoc that treats the hr as an empty header and causes Flatdoc's `slugify` to throw an error.
- Remove superfluous, `<a name>` elements from headers, GitHub already gives them a link
- Use Swift orange instead of teal for links