input: Write a function to count the frequency of words.
  assert WordFrequency().add_word("apple") == None
  assert WordFrequency().add_word("banana") == None
  assert WordFrequency().add_word("apple") == None
  assert WordFrequency().words == {"apple": 2, "banana": 1}
output: class WordFrequency:
      def __init__(self):
          self.words = {}
      
      def add_word(self, word):
          if word in self.words:
              self.words[word] += 1
          else:
              self.words[word] = 1