rrdtool: add source code URL, license, language

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

README.md

@@ -540,7 +540,7 @@ _See also: [Databases](#databases)_
 - [Diamond](https://github.com/python-diamond/Diamond) - Daemon that collects system metrics and publishes them to Graphite (and others). `MIT` `Python`
 - [Grafana](https://grafana.com/) - A Graphite & InfluxDB Dashboard and Graph Editor. ([Source Code](https://github.com/grafana/grafana)) `AGPL-3.0` `Go`
 - [Graphite](https://graphite.readthedocs.org/en/latest/) - Scalable graphing server. ([Source Code](https://github.com/graphite-project/graphite-web)) `Apache-2.0` `Python`
-- [RRDtool](https://oss.oetiker.ch/rrdtool/) - Industry standard, high performance data logging and graphing system for time series data.
+- [RRDtool](https://oss.oetiker.ch/rrdtool/) - Industry standard, high performance data logging and graphing system for time series data. ([Source Code](https://github.com/oetiker/rrdtool-1.x)) `GPL-2.0` `C`
 - [Statsd](https://github.com/etsy/statsd/) - Application statistic listener.
 - [tcollector](http://opentsdb.net/docs/build/html/user_guide/utilities/tcollector.html) - System statistic collection daemon written in Python for OpenTSDB
 - [Telegraf](https://github.com/influxdata/telegraf) - The plugin-driven server agent for collecting & reporting metrics.