Update CI badge in README.md

Author: lucassaldanha

README.md

@@ -1,6 +1,6 @@
 # discovery
 
-[![CircleCI](https://dl.circleci.com/status-badge/img/gh/Consensys/discovery/tree/master.svg?style=svg)](https://dl.circleci.com/status-badge/redirect/gh/Consensys/discovery/tree/master)
+[![ci](https://github.com/Consensys/discovery/actions/workflows/ci.yml/badge.svg)](https://github.com/Consensys/discovery/actions/workflows/ci.yml)
 [![Latest version of 'discovery' @ Cloudsmith](https://api-prd.cloudsmith.io/v1/badges/version/consensys/maven/maven/discovery/latest/a=noarch;xg=tech.pegasys.discovery/?render=true&show_latest=true)](https://cloudsmith.io/~consensys/repos/maven/packages/detail/maven/discovery/latest/a=noarch;xg=tech.pegasys.discovery/)
 
 ## Overview