added docker run command to readme (#2491)

* added docker run command to readme

* updated codebox in readme

README.md

@@ -49,6 +49,12 @@ Debian packages [are available](https://packages.debian.org/sid/net/prometheus).
 
 Docker images are available on [Quay.io](https://quay.io/repository/prometheus/prometheus).
 
+You can launch a Prometheus container for trying it out with
+
+    $ docker run --name prometheus -d -p 127.0.0.1:9090:9090 quay.io/prometheus/prometheus
+
+Prometheus will now be reachable at http://localhost:9090/.
+
 ### Building from source
 
 To build Prometheus from the source code yourself you need to have a working