Add a help text to the Slack Service

When setting up the service I was not sure which webhook I should use, by adding
this Help text I hope we can help people in the future with setting up this hook

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

app/models/project_services/slack_service.rb

@@ -34,6 +34,12 @@ class SlackService < Service
     'slack'
   end
+  def help
+    'This service sends notifications to your Slack channel.<br/>
+    To setup this Service you need to create a new <b>"Incoming webhook"</b> in your Slack integration panel,
+    and enter the Webhook URL below.'
+  end
+
   def fields
     [
       { type: 'text', name: 'webhook',