cacti: add source code link, license, language

- ref. https://github.com/awesome-foss/awesome-sysadmin/issues/416

README.md

@@ -503,7 +503,7 @@ _See also: [IT Asset Management]([Ralph](#it-asset-management))_
 - [Bloonix](https://bloonix-monitoring.org/) - Bloonix is a monitoring solution that helps businesses to ensure high availability and performance. ([Source Code](https://github.com/bloonix)) `GPL-3.0` `Perl`
 - [Bosun](http://bosun.org/) - Monitoring and alerting system by Stack Exchange ([Source Code](https://github.com/bosun-monitor/bosun)) `MIT` `Go`
 - [Cabot](http://cabotapp.com/) - Monitoring and alerts, similar to PagerDuty. ([Source Code](https://github.com/arachnys/cabot)) `MIT` `Python`
-- [Cacti](http://www.cacti.net) - Web-based network monitoring and graphing tool.
+- [Cacti](http://www.cacti.net) - Web-based network monitoring and graphing tool. ([Source Code](https://github.com/Cacti/cacti)) `GPL-2.0` `PHP`
 - [cadvisor](https://github.com/google/cadvisor) - Analyzes resource usage and performance characteristics of running containers ([Source Code](https://github.com/google/cadvisor)) `Apache-2.0` `Go`
 - [checkmk](https://checkmk.com/) - Comprehensive solution for monitoring of applications, servers, and networks. ([Source Code](https://github.com/tribe29/checkmk)) `Python/PHP`
 - [Dash](https://github.com/afaqurk/linux-dash) - A low-overhead monitoring web dashboard for a GNU/Linux machine.