docs(README): fix installation commands for marketplace

Use correct commands:
- `/plugin marketplace add` to add the marketplace
- `/plugin install plugin-name@marketplace-name` to install

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sjnims

README.md

@@ -195,10 +195,11 @@ Use this workflow for structured, high-quality plugin development from concept t
 
 ## Installation
 
-Install from this repository:
+Add this marketplace and install the plugin:
 
 ```bash
-/plugin install sjnims/plugin-dev
+/plugin marketplace add sjnims/plugin-dev
+/plugin install plugin-dev@sjnims/plugin-dev
 ```
 
 Or for development, use directly: