README: Add mention to which client it uses

Author: lilic

README.md

@@ -1,7 +1,10 @@
 # Prometheus Example App
 
-This example app serves as an example of how one can easily instrument HTTP handlers with [Prometheus](https://prometheus.io/) metrics.
+This example app serves as an example of how one can easily instrument HTTP handlers with [Prometheus][prometheus] metrics. It uses the Prometheus [go client][client-golang] to create a new Prometheus registry.
 
 Usage is simple, on any request to `/` the request will result in a `200` response code. This increments the counter for this response code. Similarly the `/err` endpoint will result in a `404` response code, therefore increments that respective counter.
 
 A Docker image is available at: `quay.io/brancz/prometheus-example-app:v0.1.0`
+
+[prometheus]:https://prometheus.io/
+[client-golang]:https://github.com/prometheus/client_golang