Updates README

- Changes heading levels, adds headings for examples
- Breaks up rationale into two paragraphs
- Changes _Catch-all_ to [_HTML5 mode_, which is Angular’s term](https://docs.angularjs.org/guide/$location) for Push State / History API that the
200.html file helps with
- Clarify you 404.html pages are custom
- Mentions SSGs and expands on first example
- Corrects `sarge.sh` to `surge.sh`