update slack setup

doc/slack_setup.md

@@ -2,13 +2,12 @@
 
 - [Create a Slack app](https://api.slack.com/apps/new) if you don't have one already
 - Pick a name, choose a workspace to associate your app with, and then click **Create App**
-- Select the **Incoming Webhooks** feature and click the **Activate Incoming Webhooks** toggle to switch it on
-- Click **Add New Webhook to Workspace**
-- Pick the channel the app will post to and then click to **Authorize** your app
-- You should see a new entry under the Webhook URLs for Your Workspace section
-
-See the [Slack API docs](https://api.slack.com/messaging/webhooks) for more detailed instructions on creating Slack apps.
+- Go to "OAuth and Permissions"
+- Add the `chat:write` permission to the app
+- Create an OAuth token for the app, this will be used as `SLACK_TOKEN`
+- By default it will post to #general. In Slack, you must invite your new bot to the channel so that it can post.
 
 - **Set environment variables in `.env`**
   - Set `ENABLE_SLACK_POSTING` to **true**
-  - Set `SLACK_WEBHOOK_URL` to the Webhook URL from your Slack app setting
+  - Set `SLACK_TOKEN` to the OAuth token for the app.
+  - Optionally, set `SLACK_CHANNEL`