docs: add Upload-Post for publishing generated clips

[mutonby]

Summary

This PR adds documentation for publishing generated clips to TikTok, YouTube Shorts, and Instagram Reels using Upload-Post.

Changes

• Added 'Publishing Your Clips' section before Contributing
• Shows how to upload clips to multiple platforms with a single API call

Why?

ClippedAI generates ready-to-upload clips with viral titles, but users still need to manually upload to each platform. Upload-Post completes the automation by providing a single API to publish to TikTok, YouTube, and Instagram simultaneously.

Example

curl -X POST https://api.upload-post.com/upload \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -F "video=@output/clip_001.mp4" \
  -F "title=Your AI-Generated Title" \
  -F "platforms=tiktok,youtube,instagram"

This complements the existing clip generation workflow by adding the final step: distribution.