feat: add logo to README

- Add docs/images directory structure for documentation assets
- Include slack-github-threads-logo-white.png logo with white background
- Update README.md to display logo prominently after project header
- Logo positioned centrally with 300px width for optimal display
- White background ensures visibility across GitHub light/dark themes

Author: markhallen

README.md

@@ -4,7 +4,7 @@
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 
 <p align="center">
-  <img src="docs/images/slack-github-threads-logo.png" alt="Slack GitHub Threads Logo" width="300" style="background-color: white; padding: 20px; border-radius: 10px;">
+  <img src="docs/images/slack-github-threads-logo-white.png" alt="Slack GitHub Threads Logo" width="300">
 </p>
 
 A Slack slash command integration that posts Slack thread conversations as comments to GitHub issues.