include cost info for Slack app (#1005)

Author: ethanpalm

ai/slack-app.mdx

@@ -10,6 +10,8 @@ icon: "slack"
 
 The Slack app adds a bot named `@mintlify` to your Slack workspace that can search your documentation and answer users' questions. The bot responds to direct messages, @mentions, and to any questions in a channel specifically named `#ask-ai`.
 
+The Slack app can incur costs: either using your AI chat credits or incurring overages.
+
 ## Setting up the Slack app
 
 <Note>