Update Slack docs to correct usage

The current docs are wrong and very unclear at how to set up Slack notifications with Envoy. I've updated the docs to the old one which was present in the Envoy readme on Github which is much more clear.

envoy.md

@@ -157,8 +157,22 @@ This is an amazingly simple way to keep your team notified of the tasks being ru
 The following syntax may be used to send a notification to [Slack](https://slack.com):
 
  @after
-  @slack('team', 'token', 'channel')
+  @slack('hook', 'channel', 'message')
  @endafter
+
+You may retrieve your token by creating an `Incoming WebHooks` integration on Slack's website.
+
+The hook argument is the entire webhook URL provided by the Incoming Webhooks Slack Integration:
+
+- `https://hooks.slack.com/services/ZZZZZZZZZ/YYYYYYYYY/XXXXXXXXXXXXXXX`
+
+You may provide one of the following for the channel argument:
+
+- For a regular channel: `#channel`
+- For a specific user: `@user`
+- If no argument is provided Envoy will use the default channel configured on the Slack website.
+
+> Note: Slack notifications will only be sent if all tasks complete successfully.
 
 <a name="envoy-updating-envoy"></a>
 ## Updating Envoy