input: Write a python function to add a team's rating to a DataFrame and ensure the rating is between 1 and 5.
  assert add_rating('Team A', 3) is None
  assert add_rating('Team B', 6) == 'Error: Rating must be between 1 and 5!'
  assert add_rating('Team C', 5) is None
output: import pandas as pd
  
  df = pd.DataFrame(columns=['Team', 'Rating'])
  
  def add_rating(team, rating):
      global df
      if 1 <= rating <= 5:
          df = df.append({'Team': team, 'Rating': rating}, ignore_index=True)
      else:
          return 'Error: Rating must be between 1 and 5!'