docs(config): add before/after hook info to dev-server (#1608)

Author: shellscape

src/content/configuration/dev-server.md

@@ -46,6 +46,18 @@ W> Be aware that when [exporting multiple configurations](/configuration/configu
 
 T> If you're having trouble, navigating to the `/webpack-dev-server` route will show where files are served. For example, `http://localhost:9000/webpack-dev-server`.
 
+## `devServer.after`
+
+`function`
+
+Provides the ability to execute custom middleware after all other middleware
+internally within the server.
+
+```js
+after(app){
+  // do fancy stuff
+}
+```
 
 ## `devServer.allowedHosts`
 
@@ -80,6 +92,21 @@ To use this option with the CLI pass the `--allowed-hosts` option a comma-delimi
 webpack-dev-server --entry /entry/file --output-path /output/path --allowed-hosts .host.com,host2.com
 ```
 
+## `devServer.before`
+
+`function`
+
+Provides the ability to execute custom middleware prior to all other middleware
+internally within the server. This could be used to define custom handlers, for
+example:
+
+```js
+before(app){
+  app.get('/some/path', function(req, res) {
+    res.json({ custom: 'response' });
+  });
+}
+```
 
 ## `devServer.bonjour`
 
@@ -666,6 +693,8 @@ webpack-dev-server --quiet
 
 `function`
 
+W> This option is __deprecated__ in favor of `before` and will be removed in v3.0.0.
+
 Here you can access the Express app object and add your own custom middleware to it.
 For example, to define custom handlers for some paths: