Add a badge of GitHub Actions

Author: mrkn

README.md

@@ -5,6 +5,7 @@
 
 # PyCall: Calling Python functions from the Ruby language
 
+[![Build Status](https://github.com/mrkn/pycall.rb/workflows/CI/badge.svg)](https://github.com/mrkn/pycall.rb/actions?query=workflow%3ACI)
 [![Build Status](https://travis-ci.org/mrkn/pycall.rb.svg?branch=master)](https://travis-ci.org/mrkn/pycall.rb)
 [![Build status](https://ci.appveyor.com/api/projects/status/0fad23u4qj1yr49e/branch/master?svg=true)](https://ci.appveyor.com/project/mrkn/pycall-rb/branch/master)