Changes:
- Fixed https://github.com/opencv/cvat/issues/14 (docker-compose down command as written in the readme does not remove volumes)
- Fixed https://github.com/opencv/cvat/issues/15 (All checkboxes in temporary attributes are checked when reopening job after saving the job)
- Fixed https://github.com/opencv/cvat/issues/18 (Extend contributing.md)
- Fixed https://github.com/opencv/cvat/issues/19 (Using the same attribute for label twice -> Stuck)