Add CI badge to README

Author: jechol

README.md

@@ -1,5 +1,7 @@
 # GlobalSemaphore
 
+[![CI](https://github.com/devall-org/global_semaphore/actions/workflows/ci.yml/badge.svg)](https://github.com/devall-org/global_semaphore/actions/workflows/ci.yml)
+
 Cluster-wide semaphore with a dead-simple API.
 
 ```elixir