Adds Buildkite badge to README.md

Author: picandocodigo

README.md

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 # Elasticsearch Serverless Client
 
 [![main](https://github.com/elastic/elasticsearch-serverless-ruby/actions/workflows/tests.yml/badge.svg?branch=main)](https://github.com/elastic/elasticsearch-serverless-ruby/actions/workflows/tests.yml)
+[![Build status](https://badge.buildkite.com/18738fa6534c59b2cd35d8ffc60a1b2fe192ab54f86c778026.svg)](https://buildkite.com/elastic/elasticsearch-serverless-ruby)
 
 This is the official Elastic client for the **Elasticsearch Serverless** service. If you're looking to develop your Ruby application with the Elasticsearch Stack, you should look at the [Elasticsearch Client](https://github.com/elastic/elasticsearch-ruby) instead. If you're looking to develop your Ruby application with Elastic Enterprise Search, you should look at the [Enterprise Search Client](https://github.com/elastic/enterprise-search-ruby/).