Added Gluster exporter in storage section (#625)

I wrote an exporter for gluster fs, its still beta but in active development. 
Check it out at https://github.com/ofesseler/gluster_exporter

content/docs/instrumenting/exporters.md

@@ -56,6 +56,7 @@ wide variety of JVM-based applications, for example [Kafka](http://kafka.apache.
 ### Storage
    * [Ceph exporter](https://github.com/digitalocean/ceph_exporter)
    * [ScaleIO exporter](https://github.com/syepes/sio2prom)
+   * [Gluster exporter](https://github.com/ofesseler/gluster_exporter)
 
 ### HTTP
    * [Apache exporter](https://github.com/neezgee/apache_exporter)