Webhooks integration documentation

As in the Readme, you integrate webhooks by adding this to the config
```
  "webhooks": {
    "#discord-channel": "https://discord.com/api/webhooks/id/token"
  }
```

However, if you mapped the channels by using the discord channel ID it's not working correctly. Only by using the channel ID in the webhooks mapping as well.

JFYI since I was stumped why it wasn't working for me :)