Update README.md: add build badge (#7)

Author: PetrHeinz

README.md

@@ -3,6 +3,7 @@
 [![Better Stack dashboard](https://github.com/logtail/logtail-aws-lambda/assets/10008612/781bb2a9-0b13-40d0-9c77-270736b10fad)](https://betterstack.com/logs)
 
 [![MIT License](https://img.shields.io/badge/license-MIT-ff69b4.svg)](./LICENSE)
+[![Build](https://github.com/logtail/logtail-aws-lambda/actions/workflows/build.yml/badge.svg)](https://github.com/logtail/logtail-aws-lambda/actions/workflows/build.yml)
 
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