message: Update design-circular-deque.py
diff: # getRear(): Gets the last item from Deque. If the deque is empty, return -1.
  # isEmpty(): Checks whether Deque is empty or not.
  # isFull(): Checks whether Deque is full or not.
  +#
  # Example:
  #
  # MyCircularDeque circularDeque = new MycircularDeque(3); // set the size to be 3
  