Write a function called add_dictionaries that takes two dictionaries with string keys and int values. It should return the result of combining the dictionaries, adding the int values together if both dictionaries have an entry with the same key. Do not modify either dictionary passed to the function. For example, given {'a': 1, 'b': 5} and {'a': 3, 'c': 6}, you would return {'a': 4, 'b': 5, 'c': 6}. Notice how the 1 and 3 values for 'a' were added together to produce 4.