docs: example for lighthouse using headers

docs/documentation/sitespeed.io/lighthouse/index.md

@@ -53,10 +53,17 @@ module.exports = {
 };
 ```
 
-You can also add Lighthouse flags by a JSON file ```--lighthouse.flags flag.json```.
+You can also add Lighthouse flags by a JSON file ```--lighthouse.flags flag.json```. If you pass on command like flags that contains hyphens, they are removed and converted internally in Lighthouse, so for example to get the command line flag `--extra-headers` to work, the JSON should be like this:
+
+```JSON
+{
+    "extraHeaders": { "key": "value"} 
+}
+```
 
 Read all about configuring Lighthouse at [https://github.com/GoogleChrome/lighthouse/blob/master/docs/configuration.md](https://github.com/GoogleChrome/lighthouse/blob/master/docs/configuration.md).
 
+
 ## Disable GPSI
 If you only want to run Lighthouse and not GPSI you can disable it with `----plugins.remove @sitespeed.io/plugin-gpsi`.