Create the third-party integrations documentation

The third-party-integrations.md file is based on the online version at:
https://docs.cachethq.io/docs/addons

docs/third-party-integrations.md

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+# Third-party Integrations
+
+Below is a list of known applications or services using the Cachet API.
+
+Name|Link|Description
+----|----|-----------
+Cachet Monitor|[https://github.com/castawaylabs/cachet-monitor][1]|For URL monitoring. Automatic incident updates
+Sensu Cachet|[https://github.com/bimlendu/sensu-cachethq][2]|Sensu handler for updating Cachet
+Hubot Cachet|[https://github.com/willdurand/hubot-cachet][3]|A hubot script to manage incidents/statuses with Cachet
+Nagios Notification|[https://github.com/mpellegrin/nagios-eventhandler-cachet][4]|A Nagios event handler to push Nagios notifications to Cachet API
+Salt Cachet|[https://github.com/alkivi-sas/salt-cachet][5]|Salt module to use with Cachet
+Pingometer|[https://pingometer.com][6]|Website uptime monitoring
+Cachet URL Monitor|[https://github.com/mtakaki/cachet-url-monitor][7]/|Monitors an URL using HTTP status code, latency, and/or payload regex
+Zabbix-Cachet|[https://github.com/qk4l/zabbix-cachet][8]|Synhronise your Zabbix IT Services and Cachet
+CheckItOn.Us|[https://checkiton.us][9]|Server monitoring tool
+Cachet-Monitor|[https://github.com/gaz492/cachet-monitor][10]|Monitors URL status via checking status codes, supports all Cloudflare http codes and more. See README for more
+
+
+[1]: https://github.com/castawaylabs/cachet-monitor
+[2]: https://github.com/bimlendu/sensu-cachethq
+[3]: https://github.com/willdurand/hubot-cachet
+[4]: https://github.com/mpellegrin/nagios-eventhandler-cachet
+[5]: https://github.com/alkivi-sas/salt-cachet
+[6]: https://pingometer.com/
+[7]: https://github.com/mtakaki/cachet-url-monitor/
+[8]: https://github.com/qk4l/zabbix-cachet
+[9]: https://checkiton.us/
+[10]: https://github.com/gaz492/cachet-monitor
+
+