docs: update README to include public folder requirement for Vercel

Author: ayclqt

vercel/README.md

@@ -19,6 +19,9 @@ This project provides a starting point for deploying a Go Fiber application to V
 - [Go](https://golang.org/dl/) 1.18 or higher
 - [Git](https://git-scm.com/downloads)
 - [Vercel CLI](https://vercel.com/download)
+- A folder named [public](public) with one or more static files to be served
+> [!CAUTION]
+> If you don't have a `public` folder, Vercel will serve all files in project, even the source code.
 
 ## Project Structure