clean migration files && add dependencies in readme

README.md

djangoldp_polls/migrations/0001_initial.py

djangoldp_polls/migrations/0002_auto_20200617_1440.py

djangoldp_polls/migrations/0003_poll_debate.py

djangoldp_polls/migrations/0004_remove_poll_debate.py

djangoldp_polls/migrations/0005_poll_startdate.py

djangoldp_polls/migrations/0006_auto_20200723_1139.py

djangoldp_polls/migrations/0007_auto_20200723_2133.py

djangoldp_polls/models.py