Updates README

- Changes heading levels, adds headings for examples
- Breaks up rationale into two paragraphs
- Clarifies features
-- HTML5 mode is Angular’s term 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
Merge pull request #1 from kennethormandy/master

Updates README