input: Write a python function to respond to the `/start` command in a Telegram bot, welcoming the user and listing available commands. 
  assert send_welcome('start') == 'Welcome! Here are the commands this bot supports:\n/list - list all items\n/add - add a new item\n/remove - remove an item\n'
  assert send_welcome('hello') == None
output: def send_welcome(command):
      if command == 'start':
          return ('Welcome! Here are the commands this bot supports:\n'
                  '/list - list all items\n'
                  '/add - add a new item\n'
                  '/remove - remove an item\n')
      return None