input: Write a python function to manage a calendar with events.
  assert calendar.events == {"04-04-2020": ["Jaunch conference"], "10-05-2020": ["Mothers Day"]}
  calendar.add_event("04-04-2020", "Project Deadline")
  assert calendar.events == {"04-04-2020": ["Jaunch conference", "Project Deadline"], "10-05-2020": ["Mothers Day"]}
  calendar.add_event("12-06-2020", "Fathers Day")
  assert calendar.events == {"04-04-2020": ["Jaunch conference", "Project Deadline"], "10-05-2020": ["Mothers Day"], "12-06-2020": ["Fathers Day"]}
output: class Calendar:
      def __init__(self):
          self.events = {}
  
      def add_event(self, date, event):
          if date in self.events:
              self.events[date].append(event)
          else:
              self.events[date] = [event]
  
  calendar = Calendar()
  calendar.add_event("04-04-2020", "Jaunch conference")
  calendar.add_event("10-05-2020", "Mothers Day")