Notify a Slack #channel when you join a room.

Setup

1. Go to your Slack app's incoming webhooks page: https://YOUR_WORKSPACE.slack.com/apps/A0F7XDUAZ-incoming-webhooks.
2. Click "Add to Slack".
3. Choose the channel where notifications should be posted, and click "Add Incoming Webhooks Integration".
4. Copy the webhook URL provided. This URL is used to post messages from external sources into Slack.

In the room-joined script replace the SLACK_WEBHOOK_URL variable with the new webhook URL you generated above.