Restructure README heading levels

This adjust the heading levels to more accurately group related content.

It ends up with the following structure:

- ember-cli
- Community
- Warning
- Why?
  - Rationale for \#3
- Guidelines
- Usage
  - Getting Started
  - Current Commands
  - Default Flags
  - LESS, Sass, or Stylus
  - Coffeescript
- Developing
  - Running Tests
- Contributing
- Ideas
- License