input: Write a python function to read a CSV file and return its content as a list of strings.
  assert read_csv('validfile.csv') == ['header1,header2\n', 'value1,value2\n']
  assert read_csv('nonexistentfile.csv') == "File not found. Please provide a valid filename"
output: def read_csv(filename):
      try:
          with open(filename, 'r') as file:
              return file.readlines()
      except FileNotFoundError:
          return "File not found. Please provide a valid filename"