add Datadog to monitoring section

I had originally intended to submit a change to the README to include the longform monitoring guides from Datadog.
Per this comment: https://github.com/n1trux/awesome-sysadmin/pull/184#issuecomment-250964295
I initially tried to append the new information to the existing entry, but a global grep found no occurrence of Datadog in the current awesome list.

README.md

@@ -388,6 +388,7 @@
   * [cadvisor](https://github.com/google/cadvisor) - Analyzes resource usage and performance characteristics of running containers ([Source Code](https://github.com/google/cadvisor)) `Apache` `Go`
   * [check_mk](http://mathias-kettner.com/check_mk.html) - Collection of extensions for Nagios.
   * [Dash](https://github.com/afaqurk/linux-dash) - A low-overhead monitoring web dashboard for a GNU/Linux machine.
+  * [Datadog](https://github.com/DataDog/dd-agent) - Real-time monitoring, visualization, and alerting as a service \[[1](https://datadoghq.com/blog?ref=awesome)\]   
   * [Flapjack](http://flapjack.io/) - Monitoring notification routing & event processing system.
   * [Healthchecks](https://healthchecks.io/) - Monitoring for cron jobs, background services and scheduled tasks.
   * [Icinga](https://www.icinga.org/) - Nagios fork that has since lapped nagios several times. Comes with the possibility of clustered monitoring - ([Source Code](https://github.com/Icinga)) - `GPLv2`