input: Write a python function to count the frequency of words in a list of words.
  assert word_frequency(['Hello', 'World', 'Hello', 'again']) == {'Hello': 2, 'World': 1, 'again': 1}
  assert word_frequency(['apple', 'banana', 'apple', 'apple', 'banana', 'cherry']) == {'apple': 3, 'banana': 2, 'cherry': 1}
  assert word_frequency(['one', 'one', 'two', 'three', 'three', 'three']) == {'one': 2, 'two': 1, 'three': 3}
output: def word_frequency(words):
      frequency = {}
      for word in words:
          if word in frequency:
              frequency[word] += 1
          else:
              frequency[word] = 1
      return frequency