Add status badge

Author: reakaleek

README.md

@@ -1,5 +1,7 @@
 # docs-builder
 
+[![PR](https://github.com/elastic/docs-builder/actions/workflows/ci.yml/badge.svg?branch=main&event=push)](https://github.com/elastic/docs-builder/actions/workflows/ci.yml)
+
 You've reached the home of the latest incarnation of the documentation tooling.
 
 This repository is host to: