Clarify thumbnail must be in plugin repository

README.md

@@ -29,9 +29,11 @@ Example: `plugins/litruv-example-plugin.json`
 **Optional fields:** `homepage`, `thumbnail`, `tags`
 
 **Thumbnail requirements:**
+- Must be a file in your plugin repository (e.g., `thumbnail.png`)
 - PNG, JPG, or GIF format
 - Max 512x512 pixels
 - Max 2MB file size
+- URL format: `http://synbox.ruv.wtf:8418/{username}/{repo}/raw/branch/main/thumbnail.png`
 
 **Important:**
 - Filename must start with your username: `{username}-`