Update Readme to clarify Slack target configuration

README.md

@@ -103,6 +103,9 @@ To generate a "Service API Key", see [PagerDuty Support: Adding Services](https:
 * `GRAPHITE_CARBON_PICKLE_PORT` - The TCP server port. Default: `2004`
 
 ##### [Slack](https://www.slack.com)
+
+The target for a Slack subscription will be the channel name (including the `#`, for example `#channel`). You can optionally suffix the channel name with `!` and that will cause the alerts to include a `@channel` mention (for example `#channel!`).
+
 * `SLACK_TOKEN` - The Slack api auth token. Default: ``
 * `SLACK_USERNAME` - The username that messages will be sent to slack. Default: `Seyren`
 * `SLACK_ICON_URL` - The user icon URL. Default: ``