title: on_reaction_add not being run
body: <p>I'm new to discord.py and trying to make a translator bot. When the user reacts with a certain flag, the bot translates it, but the event is never getting called hence I have no code to translate any messages yet. I know it's not getting called because the program isn't printing an <code>'x'</code> to the console.</p>
  
  <pre><code>@client.event
  async def on_reaction_add(reaction, user):
      channel = reaction.message.channel
      print('x')
      await client.send_message(channel, '{} has added {} to the the message {}'.format(user.name, reaction.emoji, reaction.message.content))
  
      await client.process_commands(reaction.message)
  </code></pre>