[docs] add FAQ for Vite middleware

Author: benmccann

documentation/faq/80-integrations.md

@@ -65,6 +65,34 @@ onMount(() => {
 
 Put the code to query your database in [endpoints](/docs#routing-endpoints) - don't query the database in .svelte files. You can create a `db.js` or similar that sets up a connection immediately and makes the client accessible throughout the app as a singleton. You can execute any one-time setup code in `hooks.js` and import your database helpers into any endpoint that needs them.
 
+### How use middleware?
+
+In dev, you can add middleware to Vite by using a Vite plugin. For example:
+
+```js
+const myPlugin = {
+  name: 'log-request-middleware',
+  configureServer(server) {
+    server.middlewares.use((req, res, next) => {
+      console.log(`Got request ${req.url}`);
+      next();
+    })
+  }
+}
+
+/** @type {import('@sveltejs/kit').Config} */
+const config = {
+	kit: {
+		target: '#svelte',
+		vite: {
+			plugins: [ myPlugin ]
+		}
+	}
+};
+
+export default config;
+```
+
 ### Does it work with Yarn 2?
 
 Sort of. The Plug'n'Play feature, aka 'pnp', is broken (it deviates from the Node module resolution algorithm, and [doesn't yet work with native JavaScript modules](https://github.com/yarnpkg/berry/issues/638) which SvelteKit — along with an [increasing number of packages](https://blog.sindresorhus.com/get-ready-for-esm-aa53530b3f77) — uses). You can use `nodeLinker: 'node-modules'` in your [`.yarnrc.yml`](https://yarnpkg.com/configuration/yarnrc#nodeLinker) file to disable pnp, but it's probably easier to just use npm or [pnpm](https://pnpm.io/), which is similarly fast and efficient but without the compatibility headaches.