docs: example for flags

README.md

@@ -50,7 +50,14 @@ export default config = {
 };
 ```
 
-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).