beats: add source code URL, license, language

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

README.md

@@ -535,7 +535,7 @@ _See also: [awesome-selfhosted/Database Management](https://github.com/awesome-s
 
 _See also: [Databases](#databases)_
 
-- [Beats](https://www.elastic.co/beats/) - Single-purpose data shippers that send data from hundreds or thousands of machines and systems to Logstash or Elasticsearch.
+- [Beats](https://www.elastic.co/beats/) - Single-purpose data shippers that send data from hundreds or thousands of machines and systems to Logstash or Elasticsearch. ([Source Code](https://github.com/elastic/beats)) `Apache-2.0`
 - [Collectd](https://collectd.org/) - System statistic collection daemon.
 - [Diamond](https://github.com/python-diamond/Diamond) - Python based statistic collection daemon.
 - [Facette](https://facette.io) - Time series data visualization and graphing software written in Go.