docs: install the Slack plugin from its Claude Code marketplace (#23)

Author: mwbrooks

README.md

@@ -15,9 +15,12 @@ Install the [Slack MCP and Skills Plugin](https://docs.slack.dev/ai/slack-skills
 Run this in a Claude Code session:
 
 ```
-/plugin install slack@claude-plugins-official
+/plugin marketplace add slackapi/slack-skills-plugin
+/plugin install slack@slack
 ```
 
+**Enable auto-updates**: `/plugin` > Marketplaces > `slack` > Enable auto-update.
+
 ### Codex
 
 Run this in your terminal: