Update documentation to note that SLACK_WEBHOOK_URL is will override …

…SLACK_TOKEN if both are specified

README.md

@@ -109,11 +109,12 @@ To generate a "Service API Key", see [PagerDuty Support: Adding Services](https:
 
 ##### [Slack](https://www.slack.com)
 
-You can specify either `SLACK_TOKEN` (which will be evaluated first for compatibility reason) or `SLACK_WEBHOOK_URL` (which should be the preferred method).
 If you set `SLACK_TOKEN`, 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!`). If you set `SLACK_WEBHOOK_URL`, you don't need to suffix the channel, you can simply use `#channel` or `@channel`.
 
-* `SLACK_TOKEN` - The Slack api auth token. Default: ``
-* `SLACK_WEBHOOK_URL` - The Slack webhook URL. Default: ``
+Slack notifications can be sent via [Incoming Webhooks](https://api.slack.com/incoming-webhooks) (preferred) or the [Slack Web API](https://api.slack.com/web).
+
+* `SLACK_TOKEN` - The Slack web api auth token. Default: ``
+* `SLACK_WEBHOOK_URL` - The Slack webhook URL. If specified, `SLACK_TOKEN` will be ignored. Default: ``
 * `SLACK_USERNAME` - The username that messages will be sent to slack. Default: `Seyren`
 * `SLACK_ICON_URL` - The user icon URL. Default: ``
 * `SLACK_EMOJIS` - Mapping between state and emojis unicode. Default: ``