Add example for X-Request-ID

Sometimes you want to set the request ID from the requester.
This is a example call how you can do this

website/content/middleware/request-id.md

@@ -45,3 +45,17 @@ DefaultRequestIDConfig = RequestIDConfig{
   Generator: generator,
 }
 ```
+
+## Set ID
+
+You can set the id from the requester with the `X-Request-ID`-Header
+
+*request*
+```
+curl -H "X-Request-ID: 3" --compressed -v "http://localhost:1323/?my=param"
+```
+
+*Log*
+```
+{"time":"2017-11-13T20:26:28.6438003+01:00","id":"3","remote_ip":"::1","host":"localhost:1323","method":"GET","uri":"/?my=param","my":"param","status":200, "latency":0,"latency_human":"0s","bytes_in":0,"bytes_out":13}
+```