Update README
Update README

  - provide high-level description of repository contents
  - bring ZenHub installation to the top of the readme, as it is the easiest way to see the project's current state
  - add Acknowledgements section
tnewell5/remote_retro,stride-nyc/remote_retro,stride-nyc/remote_retro,tnewell5/remote_retro,samdec11/remote_retro,stride-nyc/remote_retro