Merge pull request #429 from Rucknar/exporter_update_rancher

Adding in link to Rancher exporter to docs

content/docs/instrumenting/exporters.md

@@ -65,6 +65,7 @@ hosted outside of the Prometheus GitHub organization.
    * [PostgreSQL exporter](https://github.com/wrouesnel/postgres_exporter)
    * [PowerDNS exporter](https://github.com/janeczku/powerdns_exporter)
    * [RabbitMQ exporter](https://github.com/kbudde/rabbitmq_exporter)
+   * [Rancher exporter](https://github.com/infinityworksltd/prometheus-rancher-exporter)
    * [Redis exporter](https://github.com/oliver006/redis_exporter)
    * [RethinkDB exporter](https://github.com/oliver006/rethinkdb_exporter)
    * [Rsyslog exporter](https://github.com/digitalocean/rsyslog_exporter)