Add documentation and update README

[atdrago]

• [ ] Add documentation from YUIDoc
• [x] Update README
  • [x] Document each script
  • [x] Document contributing guidelines
  • [x] Ensure on-boarding process is smooth
  • [x] Document dependencies and versions of node that are needed (node v6 does not work)
  • [x] Move demo animation to docs/usage.md
  • [x] ~~Add small image of Negative slightly shifted over a flower 😏🌸~~
  • [x] Add blurb about downloading the app from /releases to the "User Documentation" section
  • [x] Link directly to the latest dmg
  • [x] Link to the twitter account ("follow @negativejs for updates")
  • [x] Update demo animation