Update exporters.md - Added hostapd exporter (#1327)

Add hostapd exporter to the list of Miscellaneous related exporters. This exporter monitors statistics of the Wi-Fi Access Points managed by the hostapd software. Code updated according to https://github.com/prometheus/docs/pull/1317#pullrequestreview-221185348Signed-off-by: Catalan, Miguel <miguel.catalan@i2cat.net>

content/docs/instrumenting/exporters.md

@@ -165,6 +165,7 @@ wide variety of JVM-based applications, for example [Kafka](http://kafka.apache.
    * [Dovecot exporter](https://github.com/kumina/dovecot_exporter)
    * [eBPF exporter](https://github.com/cloudflare/ebpf_exporter)
    * [Ethereum Client exporter](https://github.com/31z4/ethereum-prometheus-exporter)
+   * [Hostapd Exporter](https://stash.i2cat.net/scm/~miguel_catalan/hostapd_prometheus_exporter.git)
    * [JMeter plugin](https://github.com/johrstrom/jmeter-prometheus-plugin)
    * [Kannel exporter](https://github.com/apostvav/kannel_exporter)
    * [Kemp LoadBalancer exporter](https://github.com/giantswarm/prometheus-kemp-exporter)