docs: clarify upload preset must be unsigned

Author: strausr

README.md

@@ -22,7 +22,7 @@ npx create-cloudinary-react
 The CLI will prompt you for:
 - Project name
 - **Cloudinary cloud name** (found in your [dashboard](https://console.cloudinary.com/app/home/dashboard))
-- Upload preset (optional - required for uploads, but transformations work without it)
+- Unsigned Upload preset (optional - required for uploads, but transformations work without it)
 - AI coding assistant(s) you're using (Cursor, GitHub Copilot, Claude, etc.)
 - Whether to install dependencies
 - Whether to start dev server