Add Docker Daemon to software exposing Prometheus metrics (prometheus#949)

Author: fhemberger

content/docs/instrumenting/exporters.md

@@ -171,6 +171,7 @@ separate exporters are needed:
    * [Collectd](https://collectd.org/wiki/index.php/Plugin:Write_Prometheus)
    * [Concourse](https://concourse.ci/)
    * [CRG Roller Derby Scoreboard](https://github.com/rollerderby/scoreboard) (**direct**)
+   * [Docker Daemon](https://docs.docker.com/engine/reference/commandline/dockerd/#daemon-metrics)
    * [Doorman](https://github.com/youtube/doorman) (**direct**)
    * [Etcd](https://github.com/coreos/etcd) (**direct**)
    * [FreeBSD Kernel](https://www.freebsd.org/cgi/man.cgi?query=prometheus_sysctl_exporter&apropos=0&sektion=8&manpath=FreeBSD+12-current&arch=default&format=html)