docs: add devMiddleware example (#3806)

Author: snitin315

examples/dev-middleware/README.md

@@ -0,0 +1,45 @@
+# devMiddleware option
+
+Provide options to [webpack-dev-middleware](https://github.com/webpack/webpack-dev-middleware) which handles webpack assets.
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  // ...
+  devServer: {
+    devMiddleware: {
+      index: true,
+      headers: {
+        "X-Custom-Header": "yes",
+      },
+    },
+  },
+};
+```
+
+To run this example use the following command:
+
+```console
+npx webpack serve --open
+```
+
+## What should happen
+
+1. The script should open `http://localhost:8080/`.
+2. You should see the text on the page itself change to read `Success!`.
+3. Open the console in your browser's devtools and select the _Network_ tab.
+4. Find `main.js`. The response headers should contain `X-Custom-Header: yes`.
+
+Now update `webpack.config.js` with [`index: false`](https://github.com/webpack/webpack-dev-middleware#index), this will tell the server to not respond to requests to the root URL.
+
+Now close and restart the server with:
+
+```console
+npx webpack serve --open
+```
+
+## What should happen
+
+1. The script should open `http://localhost:8080/`.
+2. You should see the `Cannot GET /` text on the page.

examples/dev-middleware/app.js

@@ -0,0 +1,6 @@
+"use strict";
+
+const target = document.querySelector("#target");
+
+target.classList.add("pass");
+target.innerHTML = "Success!";

examples/dev-middleware/webpack.config.js

@@ -0,0 +1,18 @@
+"use strict";
+
+// our setup function adds behind-the-scenes bits to the config that all of our
+// examples need
+const { setup } = require("../util");
+
+module.exports = setup({
+  context: __dirname,
+  entry: "./app.js",
+  devServer: {
+    devMiddleware: {
+      index: false,
+      headers: {
+        "X-Custom-Header": "yes",
+      },
+    },
+  },
+});