[REBASED] Improve README

- Remove section ‘Roadmap’
- Move section ‘Usage’ to the top
- Add link to expressjs.com
- Add link to LICENSE
- This repository is using GitHub issues and milestones from now on.
  Removed bullet points are at:
    - https://github.com/sbruchmann/brackets-static-preview/issues/17
    - https://github.com/sbruchmann/brackets-static-preview/issues/18