Add IBM Z HMC exporter (#1160)

* Added IBM Z HMC exporter
Signed-off-by: Jakob-Naucke <jakob.naucke@ibm.com>

content/docs/instrumenting/exporters.md

@@ -54,6 +54,7 @@ wide variety of JVM-based applications, for example [Kafka](http://kafka.apache.
 ### Hardware related
    * [apcupsd exporter](https://github.com/mdlayher/apcupsd_exporter)
    * [Collins exporter](https://github.com/soundcloud/collins_exporter)
+   * [IBM Z HMC exporter](https://github.com/zhmcclient/zhmc-prometheus-exporter)
    * [IoT Edison exporter](https://github.com/roman-vynar/edison_exporter)
    * [IPMI exporter](https://github.com/lovoo/ipmi_exporter)
    * [knxd exporter](https://github.com/RichiH/knxd_exporter)