Add a CI badge to show the build status

Author: Pere Urbon-Bayes

README.md

@@ -1,5 +1,8 @@
 # Logstash Plugin
 
+[![Build
+Status](http://build-eu-00.elastic.co/view/LS%20Plugins/view/LS%20Outputs/job/logstash-plugin-output-tcp-unit/badge/icon)](http://build-eu-00.elastic.co/view/LS%20Plugins/view/LS%20Outputs/job/logstash-plugin-output-tcp-unit/)
+
 This is a plugin for [Logstash](https://github.com/elastic/logstash).
 
 It is fully free and fully open source. The license is Apache 2.0, meaning you are pretty much free to use it however you want in whatever way.