input: Write a python function to generate a random username by combining two words from a given list.
  assert generate_username(['cat', 'dog', 'mouse', 'elephant', 'tiger', 'lion', 'giraffe', 'hippo', 'monkey', 'zebra']).count('_') == 1
  assert generate_username(['apple', 'banana', 'cherry', 'date', 'fig', 'grape']).count('_') == 1
  assert len(generate_username(['red', 'blue', 'green', 'yellow', 'purple', 'orange'])) > 0
output: import random
  
  def generate_username(words):
      word1, word2 = random.sample(words, 2)
      return f"{word1}_{word2}"