docs: update plugin-react-refresh README.md for middleware mode

[pengx17]

While I was setting up a react app with Vite, everything works fine before enabling reactRefresh plugin.
When this is enabled, we will get a "vite-plugin-react can't detect preamble" error. After some investigation, looks like on the userland we need to explicitly transform the index file with ViteDevServer.transformIndexHtml.

Related:

• vite/packages/vite/src/node/server/index.ts

  Lines 447 to 455 in da36547

  	if (!middlewareMode) {
  	// transform index.html
  	middlewares.use(indexHtmlMiddleware(server))
  	// handle 404s
  	middlewares.use((_, res) => {
  	res.statusCode = 404
  	res.end()
  	})
  	}

• [plugin-react-refresh] auto page reloading is broken when a jsx file contain multi-components vite-plugin-react#11 (comment)