Given a dictionary d$,$ a key, and a value, this function adds the key:value pair to the dictionary d$.$

If the key doesn$$t already exist in the dictionary, this function should simply add it$.$ However, if

the key already exists, then rather than overriding the previous value, the dictionary should start

keeping a list of values for that key, in the order they are added.

Preconditions and Postconditions

d: dict $->$ You cannot make any assumptions about the size or contents of d

key: any immutable type

value: any

Returns: None