Add https://github.com/chop-dbhi/prometheus-sql to list of exporters.

content/docs/instrumenting/exporters.md

@@ -55,6 +55,7 @@ hosted outside of the Prometheus GitHub organization.
    * [Rsyslog exporter](https://github.com/digitalocean/rsyslog_exporter)
    * [scollector exporter](https://github.com/tgulacsi/prometheus_scollector)
    * [SMTP/Maildir MDA blackbox prober](https://github.com/cherti/mailexporter)
+   * [SQL query result set metrics exporter](https://github.com/chop-dbhi/prometheus-sql)
 
 ## Directly instrumentated software