Adding build badge (#4233)

Author: amitkanfer

README.md

@@ -1,5 +1,6 @@
 # Elastic Agent
 
+[![Build status](https://badge.buildkite.com/1d35bb40427cc6833979645b61ea214fc4b686a2ffe3a68bdf.svg)](https://buildkite.com/elastic/elastic-agent)
 [![Coverage](https://sonarcloud.io/api/project_badges/measure?project=elastic_elastic-agent&metric=coverage)](https://sonarcloud.io/summary/new_code?id=elastic_elastic-agent)
 
 ## Architecture / internal docs